Sound of key opening (6)
I believe the answer is:
eyelet
'opening' is the definition.
(hole for putting a shoelace through)
'sound of key' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'key' becomes 'islet' (both can mean a small island).
'islet' sounds like 'EYELET'.
(Other definitions for eyelet that I've seen before include "Small opening in needle for thread" , "Small hole for lacing" , "Small opening for needle or shoelace" , "Small hole through which lace passes" , "Small lace-hole in shoe (6)" .)