Net percentage up (4)
I believe the answer is:
trap
'net' is the definition.
(thesaurus)
'percentage up' is the wordplay.
'percentage' becomes 'part' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'part' back-to-front is 'TRAP'.
(Other definitions for trap that I've seen before include "that may catch a speeding motorist" , "Snare - mouth" , "Golfing hazard" , "Snare, ruse" , "fault" .)