Crossword Genius

Perplex indefinite number on benefit (7)

Ross

I believe the answer is:

nonplus

'perplex' is the definition.
(synonyms)

'indefinite number on benefit' is the wordplay.
'indefinite number' becomes 'n' (mathematical symbol).
'benefit' becomes 'plus' (I've seen this before).
'n'+'on'+'plus'='NONPLUS'

(Other definitions for nonplus that I've seen before include "Baffle" , "Utterly confuse" , "stump" , "Get confused" , "nothing if not negative?" .)

I've seen this clue in The Telegraph.
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