Material in ninety skirts, fifty being worn (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'ninety skirts fifty being worn' is the wordplay.
'skirts' means to remove the middle letters (retain the edges).
'fifty' becomes 'l' (roman numeral).
'being worn' becomes 'on'.
'ninety' with its centre removed is 'ny'.
'ny'+'l'+'on'='NYLON'
'in' is the link.
(Other definitions for nylon that I've seen before include "may be damaged by running" , "Tights material" , "Man-made material" , "Fibrous material" , "Artificial yarn/fabric" .)