Trashed new fibres (7)
I believe the answer is:
threads
'fibres' is the definition.
(I have seen 'Thin fibres ' mean 'threads' so perhaps 'fibres' could also mean 'threads')
'trashed new' is the wordplay.
'new' is an anagram indicator.
'trashed' anagrammed gives 'THREADS'.
(Other definitions for threads that I've seen before include "Clothes (slang)" , "Clothing" , "Fine, twisted filaments" , "Trashed (anag)" , "Fine cords of twisted fibres" .)