Small instrument for cutting (5)
I believe the answer is:
sharp
'for cutting' is the definition.
(synonyms)
'small instrument' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'instrument' becomes 'harp' (harp is a kind of instrument).
's'+'harp'='SHARP'
(Other definitions for sharp that I've seen before include "character on the staff" , "Acidic; smart and stylish" , "Not blunt" , "Cutting; note" , "Penetrating" .)