A double definition clue, as the name indicates, has two definitions for the same word. This technique is used to clue homographs – words that have the same spelling but different meanings and usually different etymology.
Double Definition Clue Structure: The clue contains two distinct parts
1. Definition 1
2. Definition 2
The two definitions may be joined together by connectors (prepositions, conjunctions, etc).
Example II (from THC 8422):
Robust author (5) HARDY 
Definition 1: Robust
Definition 2: author
Bike was blue (5) MOPED 
Definition 1: Bike
Definition 2: was blue
How To Recognize Double Definitions
You probably have a double definition on hand if:
- the clue is very short – 2 or 3 words, which may be joined together with connectors(articles, prepositions, etc.)
NIE 05-Feb-10: Business worry (6) CONCERN 
NIE 07-Apr-09: Succession of command (5) ORDER 
- the clue appears to have a separation in the middle
THC 9353: Dealer in stolen property — might be radio equipment (8) RECEIVER 
Clues with that structure are likely to be double-definitions, but that does not imply the reverse – not all double-definitions are short with discrete halves - they can be long with the two definitions blended seamlessly. This is one such:
ET 3781: Something on the inside gets one going to sea (5) LINER 
This brings us to a few more points worth keeping in mind when dealing with double definitions.
Things To Watch Out For
- The two definitions need not be straight, either or both could be cryptic definitions.
THC 9354: Exploits that require to be witnessed (5) DEEDS 
The wordplay here is on the two meanings of DEEDS, but the part "that require to be witnessed" needs to be read along with "Exploits" to arrive at the solution. This works more like a cryptic definition than a double definition. See more clues of this type under Cryptic Double-Definitions.
- Parts of speech of the solution may be different in its two definitions.
(From the Times): One making recording has to come to the point (5) TAPER 
TAPER is a noun by the first definition, a verb by the second.
- The clue could be 'multi-definition' (more than 2 definitions for the solution), though this is rare.
Guardian 24930 (Brendan): Squander money in way that's sad and obscene (4) BLUE 
'Squander money', 'sad' and 'obscene' are three different meanings of BLUE.
The definitions should ideally have disconnected meanings, but that is not always the case. The Hindu Crossword often carries double definitions of words with similar meanings or meanings are derived from the same root.
THC 9346: Eaten up and taken in eagerly (8) DEVOURED 
The neologism for such clues is duds, short for duplicate definitions.
- Unless marked by the distinct properties under 'how to recognise…', double-definition clues do not have telltale signs to give away the clue type. There is no 'double definition' indicator. The surface reading can be similar to charades or cryptic definitions. Till other possibilities are ruled out, keep an open mind about assuming that an unsolved clue is a double-definition.
NIE 20-Jun-09: Fruit goes out of fashion (5)
FT 13311 (Neo): Me or Adam? (5,6)
FT 13311 (Neo): Spy found (5)
- Other Clue Types: Anagrams, Hidden Words, Charades, Containers,
Deletions, Reversals, Homophones, Substitutions, Acrostics, Letter Picking,
Letter Shifting, Letter Exchange, Cryptic Definitions, &Lit.
- More Double Definition Clues
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