Top music group given external support (7)
I believe the answer is:
Here is my best explanation:
'top' is the definition.
'music group given external support' is the subsidiary indication.
'music group' becomes 'rem' (US band).
'given external' means one lot of letters goes inside another.
'support' becomes 'pier' (pier is a kind of support).
'rem' put inside 'pier' is 'premier'.
(Other definitions for premier that I've seen before include "Foremost", "Earliest", "Chief minister", "Leading", "Prime minister", "Aberdeen or Salisbury?", "man who's in charge", "First - head of government", "A head of government" and "PM".)