Plays, relaxing in company, a song (7)
I believe the answer is:
calypso
'a song' is the definition.
(thesaurus)
'plays relaxing in company' is the wordplay.
'relaxing' is an anagram indicator.
'in' is an insertion indicator.
'company' becomes 'co' (abbreviation).
'plays' anagrammed gives 'alyps'.
'alyps' put into 'co' is 'CALYPSO'.
(Other definitions for calypso that I've seen before include "A number of West Indians" , "W. Indies music" , "Caribbean dance" , "Caribbean satirical song" , "West Indian music style" .)