Hardest shape for 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')
'hardest shape' is the wordplay.
'shape' indicates an anagram.
'hardest' is an anagram of 'THREADS'.
'for' acts as a link.
(Other definitions for threads that I've seen before include "Fine, twisted filaments" , "Clothing" , "Fine cords of twisted fibres" , "Clothes (slang)" , "Strands for sewing" .)