Carbon in constellation's fabric (5)
I believe the answer is:
lycra
'fabric' is the definition.
(I know that Lycra is a type of fabric)
'carbon in constellation's' is the wordplay.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'in' indicates putting letters inside.
'constellation' becomes 'lyra' (Lyra is an example).
'c' going into 'lyra' is 'LYCRA'.
(Other definitions for lycra that I've seen before include "synthetic fibre" , "Synthetic fabric" , "gear for cyclists" , "skin-tight garments" , "Fabric used in tight-fitting cycling wear" .)