Take Nita Jaggi's clue from The Hindu Crossword 9649:
5 Indian state has one official language (7)
The parsing is:
Indian state = BENGAL
has = connector
one = I
Definition: official language = BENGAL + I = BENGALI
The clue has a meaningful surface, cryptic elements correctly placed and indicated. Technically all looks okay (ignore for now that the clue may have more than one correct answer). Why, then, is it a bad clue?
The essence of a cryptic clue lies in creative play on words, and in misleading/surprising the solver. None of these hold true when the subsidiary indication and the definition share the same root word.
The word BENGALI is derived from BENGAL; there's nothing very creative or exciting about splitting it into BENGAL + I.
Therefore, a weak clue. Etymologically-related constructs like (BAT + S = BATS) or (ADMIRE– E + ABLE = ADMIRABLE), when used to clue charades, are flaws in my book.
The Other Extreme
The other equally uncreative extreme is breaking the answer into the tiniest bits and combining them using abbreviations. Nita Jaggi favours this as well – refer Charade Overdose.
A sample from the same puzzle (THC 9649):
To inspire one doctor you see at last (5) I MB U E
More Weak Wordplay, What Say?
What do you think of of the clues below? Good wordplay, or not? If not good, why not?
Eaten up and taken in eagerly (8) DEVOURED 
Finally, students are able to read quickly (4) S CAN
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