Small instrument for piercing (5)
I believe the answer is:
sharp
'piercing' is the definition.
(I know that piercing can be written as sharp)
'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 within the answer.
'instrument' could be 'harp' (harp is a kind of instrument) and 'harp' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Clearly defined; acidic" , "Piercing; clearly defined" , "Musical notation of a half step higher than the note named" , "Incisive" , "Kind of musical note" .)