Widely grown but it's not used (4)
I believe the answer is:
mint
'used' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'widely grown but it's not' is the wordplay.
I cannot quite see how this works, but
'not' could be 'nt' (synonyms) and 'nt' is present in the answer.
The remaining letters 'mi' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mint that I've seen before include "Vast sum of money" , "Produce coins" , "Freshly made" , "Plant - in perfect condition" , "Culinary herb" .)