Did badly in fighting drug dependency (9)
I believe the answer is:
addiction
'drug dependency' is the definition.
(drug addiction is being dependent on drugs)
'did badly in fighting' is the wordplay.
'badly' indicates anagramming the letters.
'in' is an insertion indicator.
'fighting' becomes 'action' (as in military action).
'did' anagrammed gives 'ddi'.
'ddi' going within 'action' is 'ADDICTION'.
(Other definitions for addiction that I've seen before include "habit that's hard to break" , "The origin of dependence" , "Bug one's caught" , "Something to kick" , "hard to stop" .)