Opening in minor key on radio (6)
I believe the answer is:
eyelet
'opening' is the definition.
(hole for putting a shoelace through)
'minor key on radio' is the wordplay.
'minor key' becomes 'islet' (I've seen this before).
'on radio' indicates a 'sounds like' (homophone) clue.
'islet' sounds like 'EYELET'.
'in' is the link.
(Other definitions for eyelet that I've seen before include "Small opening for needle or shoelace" , "Hole for fastener" , "peephole" , "Small opening in shoe for lace" , "Small hole for lacing" .)