Crossword Genius

Mug — I had nothing to fill it (5)

Ross

I believe the answer is:

idiot

'mug' is the definition.
(thesaurus)

'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'.
'id'+'iot'='IDIOT'

(Other definitions for idiot that I've seen before include "Charlie" , "Ignoramus's" , "Foolish person" , "Ass, nincompoop" , "eg Dostoevski's Myshkin" .)

I've seen this clue in The Telegraph.
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