One moaning about idiot who sings when tipsy? (9)
I believe the answer is:
wassailer
'who sings when tipsy?' is the definition.
(wassailing is an archaic term for drunken revelry)
'one moaning about idiot' is the wordplay.
'one moaning' becomes 'wailer'.
'about' indicates putting letters inside.
'idiot' becomes 'ass' (I've seen this before).
'wailer' enclosing 'ass' is 'WASSAILER'.
(Other definitions for wassailer that I've seen before include "Carol singer" , "Carouser" , "One cheerful at Christmas" .)