Chatter behind gym with US stars (7)
I believe the answer is:
pegasus
'stars' is the definition.
(constellation)
'chatter behind gym with us' is the wordplay.
'chatter' becomes 'gas' (I've seen this in another clue).
'behind' means one lot of letters go next to another.
'gym' becomes 'pe' (short for Physical Education).
'with' means one lot of letters go next to another.
'gas' put after 'pe' is 'pegas'.
'pegas'+'us'='PEGASUS'
(Other definitions for pegasus that I've seen before include "Winged horse - constellation" , "mount in the clouds" , "Mythological horse" , "Mythical flying horse" , "fabulous horse" .)