Within planet, unearthly melody (4)
I believe the answer is:
'melody' is the definition.
(I know that melody can be written as tune)
'within planet unearthly' is the wordplay.
'within' indicates the answer is hidden within the clue.
'TUNE' is hidden within 'planet unearthly'.
(Other definitions for tune that I've seen before include "Adjust (car engine); air" , "Of engine, adjust to run smoothly" , "Air, melody" , "Adjust for better performance" , "strain" .)