Perplex indefinite number on benefit (7)
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?" .)