Second instrument, with directions to improve (7)
I believe the answer is:
sharpen
'second instrument' is the definition.
'sharpen' can be an answer for 'instrument' (thesaurus). I am unsure of the 'second' bit.
'directions to improve' is the wordplay.
'directions' becomes 'en' (I've seen this in other clues).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'improve' becomes 'sharp' (I can't explain this - if you can you should give a lot more credence to this answer).
'en' put after 'sharp' is 'SHARPEN'.
'with' is the link.
Can you help me to learn more?
(Other definitions for sharpen that I've seen before include "Make pointed" , "Put a keen edge on" , "Put a point on" , "Hone (a knife blade)" , "Make more acute" .)