Completely perplex small number on bonus (7)

I believe the answer is:

nonplus

'completely perplex' is the definition.
(I've seen this before)

'small number on bonus' is the wordplay.
'small number' becomes 'n' (I've seen this in other clues).
'bonus' becomes 'plus' (I've seen this before).
'n'+'on'+'plus'='NONPLUS'

(Other definitions for nonplus that I've seen before include "Surprise and confuse" , "Dumbfound" , "Get confused" , "Perplex totally" , "Utterly confuse" .)

I've seen this clue in The Telegraph.

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