Fabric a group of stars used to hem cape (5)
I believe the answer is:
lycra
'fabric' is the definition.
(I have seen 'Synthetic fabric ' mean 'lycra' so perhaps 'fabric' could also mean 'lycra')
'a group of stars used to hem cape' is the wordplay.
'a group of stars' becomes 'lyra' (Lyra is a constellation).
'used to hem' means one lot of letters goes inside another (hem can mean to enclose or surround).
'cape' becomes 'c' (abbreviation used on maps).
'lyra' enclosing 'c' is 'LYCRA'.
(Other definitions for lycra that I've seen before include "Sports clothes fabric" , "Material for exercise wear" , "synthetic fibre" , "Stretch fabric" , "Sports clothing material" .)