Not much harm in using less (5)
I believe the answer is:
minus
'less' is the definition.
(I've seen this in other clues)
'not much harm in using' is the wordplay.
'not much' says the answer is hidden in the clue (I've seen 'not much of' mean this).
'MINUS' is hidden in the letters of 'harm in using'.
(Other definitions for minus that I've seen before include "'Take away' sign" , "Less, without" , "Negative sign - negative thing" , "Reduced by the subtraction of - thus?" , "Lacking; below zero" .)