Good wedding attendant: well? (6)
I believe the answer is:
gusher
'well?' is the definition.
(gusher is a kind of well)
'good wedding attendant' is the wordplay.
'good' becomes 'g' (abbreviation).
'wedding attendant' becomes 'usher' (I've seen this before).
'g'+'usher'='GUSHER'
(Other definitions for gusher that I've seen before include "one exuding enthusiasm" , "Effusive person or oil well in action" , "Freely-flowing oil well" , "Well, maybe" , "one's over-effusive" .)