Wit in Spar after drug (6)
I believe the answer is:
esprit
'wit' is the definition.
(esprit can mean being witty)
'spar after drug' is the wordplay.
'spar' becomes 'sprit' (sprit is a kind of spar).
'after' means one lot of letters go next to another.
'drug' becomes 'e' (E is another name for ecstasy).
'sprit' after 'e' is 'ESPRIT'.
'in' acts as a link.
(Other definitions for esprit that I've seen before include "Liveliness - priest (anag)" , "Liveliness of wit" , "... de corps, team spirit" , "Sprightliness" , "Stripe (anag)" .)