Mug: I had nothing to fill it! (5)
I believe the answer is:
idiot
'mug' is the definition.
(I've seen this before)
'i had nothing to fill it' is the wordplay.
'i had' becomes 'I'd' (contraction).
'nothing' becomes 'O' (O resembles 0).
'to fill' is an insertion indicator.
'o' placed inside 'it' is 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "Charlie" , "Ignoramus's" , "Foolish person" , "Ass, nincompoop" , "eg Dostoevski's Myshkin" .)