Drugs work, I answer group in review (7)
I believe the answer is:
opiates
'drugs' is the definition.
(opiate is a kind of drug)
'work i answer group in review' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'answer' becomes 'a' (abbreviation eg in Q and A).
'group' becomes 'set' (both can mean a collection).
'in review' shows that the letters should be reversed in order.
'set' backwards is 'tes'.
'op'+'i'+'a'+'tes'='OPIATES'
(Other definitions for opiates that I've seen before include "Poppy-derived drugs" , "Sedative drugs" , "Narcotic drugs" , "Is a poet (anag.)" , "Branch of narcotics" .)