Small instrument out of tune (5)
I believe the answer is:
sharp
'out of tune' is the definition.
(I've seen this in another clue)
'small instrument' is the wordplay.
I cannot quite see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'instrument' could be 'harp' (harp is a kind of instrument) and 'harp' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(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" .)