Less in sum maybe (5)
I believe the answer is:
minus
'less' is the definition.
(I've seen this before)
'in sum maybe' is the wordplay.
'maybe' indicates an anagram.
'in'+'sum'='insum'
'insum' is an anagram of 'MINUS'.
(Other definitions for minus that I've seen before include "Disadvantage" , "Take-away (sign)" , "Lacking; negative" , "Deprived of" , "Negative sign - negative thing" .)