Offence is recalled by head of narcotics (3)
I believe the answer is:
sin
'offence' is the definition.
(a sin is an offence or transgression)
'is recalled by head of narcotics' is the wordplay.
'recalled' shows that the letters should be reversed in order (to recall can mean to bring something back).
'by' is a charade indicator (letters next to each other).
'head of' suggests taking the first letters.
The first letter of 'narcotics' is 'n'.
'is' backwards is 'si'.
'si'+'n'='SIN'
(Other definitions for sin that I've seen before include "Transgress" , "Mortal offence" , "Lapse against moral law" , "Serious error" , "Breach of moral code" .)