Crescent shape clear in old stringed instrument (7)
I believe the answer is:
lunette
'crescent' is the definition.
(I've seen this before)
'clear in old stringed instrument' is the wordplay.
'clear' becomes 'net' (as in net profit).
'in' is an insertion indicator.
'old stringed instrument' becomes 'lute' (I've seen this before).
'net' inserted within 'lute' is 'LUNETTE'.
'shape' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lunette that I've seen before include "Arched aperture" , "Crescent-shaped space" , "Towing-ring; crescent shape" , "Semi-circular alcove" , "opening in dome" .)