Top music group given external support (7)
I believe the answer is:
premier
'top' is the definition.
(thesaurus)
'music group given external support' is the wordplay.
'music group' becomes 'rem' (US band).
'given external' is an insertion indicator.
'support' becomes 'pier' (pier is a kind of support).
'rem' going within 'pier' is 'PREMIER'.
(Other definitions for premier that I've seen before include "First (minister)" , "man who's in charge" , "Foremost" , "Prime minister" , "First - head of government" .)