Small instrument with strings 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 with strings' is the wordplay.
I cannot really see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is located in the answer.
'instrument' could be 'harp' (harp is a kind of instrument) and 'harp' is present in the answer.
No letters remain.
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 "Piercing; clearly defined" , "Acute; pointed" , "exactly on time" , "Fly - biting" , "Incisive" .)