Party hard? What an idiot I am! (3)
I believe the answer is:
doh
'what an idiot i am' is the definition.
'party hard?' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'hard?' becomes 'h'.
'do'+'h'='DOH'
(Other definitions for doh that I've seen before include "How foolish of me" , "First and eighth note in tonic sol-fa" , "It begins the tonic solfa" , "for 7 [HOMER] of course!" , "First note of major scale in music" .)