Mug — I had nothing to fill it (5)
I believe the answer is:
'mug' is the definition.
'i had nothing to fill it' is the wordplay.
'i had' becomes 'id' (I'd).
'nothing' becomes 'o' (looks like zero - 0).
'to fill' is an insertion indicator.
'o' put inside 'it' is 'iot'.
(Other definitions for idiot that I've seen before include "Charlie" , "Ignoramus's" , "Foolish person" , "Ass, nincompoop" , "eg Dostoevski's Myshkin" .)