Small instrument out of tune (5)
I believe the answer is:
sharp
'out of tune' is the definition.
(thesaurus)
'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 "tart" , "British musician" , "Clearly defined; acidic" , "woman's name in Vanity Fair" , "Quick-witted - keen" .)