Very strong drug? Primarily it's found in drink (7)
I believe the answer is:
'drink' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'very strong drug? primarily it's found in' is the wordplay.
'very strong' becomes 'ff' (musical abbreviation for fortissimo, 'very strong').
'drug?' becomes 'ca' (I can't explain this - if you can you should give a lot more credence to this answer).
'primarily' is a charade indicator (letters next to each other).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'found' is a charade indicator (letters next to each other).
'ff' after 'ca' is 'caff'.
(Another definition for caffein that I've seen is " Coffee ingredient (alt. spelling)".)