Fantastic support for stars (5,4)
I believe the answer is:
great bear
'for stars' is the definition.
(constellation Ursa Major)
'fantastic support' is the wordplay.
'fantastic' becomes 'great' (fantastic can mean great or excellent).
'support' becomes 'bear' (bear can mean to support or carry).
'great'+'bear'='GREAT BEAR'
(Other definitions for great bear that I've seen before include "visible in the night sky" , "Large animal - same as 7" , "'Large ursine in the heavens (5,4)'" , "star group" , "Ursa Major" .)