Supply drug (crack) (5)
I believe the answer is:
equip
'supply' is the definition.
(I know that equip is a kind of supply)
'drug crack' is the wordplay.
'drug' becomes 'e' (E is another name for ecstasy).
'crack' becomes 'quip' (both can mean a joke).
'e'+'quip'='EQUIP'
(Other definitions for equip that I've seen before include "Provide what's needed" , "Provide with requirements" , "Supply (with items)" , "Accoutre" , "Fit out, supply" .)