Complain pathetically about idiot revelry (7)
'revelry' is the definition.
(wassail can mean to drink during a festival)
'complain pathetically about idiot' is the wordplay.
'complain pathetically' becomes 'wail' (to wail is to lament or complain).
'about' indicates putting letters inside.
'idiot' becomes 'ass' (I've seen this before).
'wail' placed around 'ass' is 'WASSAIL'.
(Other definitions for wassail that I've seen before include "festive drink", "One that may come in to bowl", "Drinking festivity", "Revelry", "Lively festivities; spiced ale".)