Catch in possession of cannabis (3)
I believe the answer is:
nab
'catch' is the definition.
(nab can mean to capture or arrest)
'in possession of cannabis' is the wordplay.
'in possession' says the answer is hidden in the clue (I've seen 'in possession of' mean this (the answer is possessed by the clue)).
'NAB' is hidden within 'of cannabis'.
(Other definitions for nab that I've seen before include "Apprehend" , "nail" , "Seize quickly" , "Collar (sl.)" , "Catch (a felon)" .)