Second instrument is out of tune (5)
I believe the answer is:
sharp
'out of tune' is the definition.
(I've seen this in another clue)
'second instrument' is the wordplay.
I cannot quite understand how this works, but
'second' could be 's' (common abbreviation) and 's' is located in the answer.
'instrument' could be 'harp' (harp is a kind of instrument) and 'harp' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Tangy" , "Cutting; acidic" , "Clearly defined (image)" , "Unscrupulous character" , "Penetrating" .)