Fine in case of bigger drug bust (5)
I believe the answer is:
broke
'bust' is the definition.
(I know that bust can be written as broke)
'fine in case of bigger drug' is the wordplay.
'fine' becomes 'ok' (synonyms).
'in' indicates putting letters inside.
'case of' suggests removing the centre (like emptying a case).
'drug' becomes 'e' (E is another name for ecstasy).
'bigger' with its centre removed is 'br'.
'br'+'e'='bre'
'ok' inserted within 'bre' is 'BROKE'.
(Other definitions for broke that I've seen before include "Bust - something to go for?" , "Ordinal number" , "See 19" , "Moneyless" , "With completely empty pockets" .)