Waste time attending clubs, perhaps in formal wear (6,4)
'formal wear' is the definition.
(I've seen this before)
'waste time attending clubs perhaps' is the wordplay.
'waste time' becomes 'lounge' (I've seen this before).
'attending' says to put letters next to each other.
'clubs perhaps' becomes 'suit' (clubs is a playing card suit).
'lounge'+'suit'='LOUNGE-SUIT'
'in' is the link.
(Other definitions for lounge suit that I've seen before include "Formal wear" , "Smart clothing" , "what smart fellow assumes" , "Attire" , "Common habit of man" .)