Cheat on husband? What an idiot I am! (3)
I believe the answer is:
doh
'what an idiot i am' is the definition.
(exclamation expressing recognition of stupidity)
'cheat on husband?' is the wordplay.
'cheat' becomes 'do'.
'on' says to put letters next to each other.
'husband?' becomes 'h'.
'do'+'h'='DOH'
(Other definitions for doh that I've seen before include "First note of musical scale" , "annoyed expression" , "Homeric grunt of annoyance" , "It begins the tonic solfa" , "The note C" .)