Small stringed instrument needing tuning? (5)
I believe the answer is:
sharp
'needing tuning?' is the definition.
'small stringed instrument' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'stringed instrument' becomes 'harp' (harp is a kind of stringed instrument).
's'+'harp'='SHARP'
(Other definitions for sharp that I've seen before include "woman's name in Vanity Fair" , "Acidic; smart and stylish" , "Musical notation of a half step higher than the note named" , "Note astute" , "music feature" .)