Aviator with tune descending on isle (6)
I believe the answer is:
airman
'aviator' is the definition.
(synonyms)
'tune descending on isle' is the wordplay.
'tune' becomes 'air' (air can mean a piece of music).
'descending on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'isle' becomes 'man' (the Isle of Man is an example).
'air'+'man'='AIRMAN'
'with' acts as a link.
(Other definitions for airman that I've seen before include "Flier" , "Male flyer" , "crew member" , "He flies" , "Aviator" .)