Clothes, hardest to repair (7)
I believe the answer is:
threads
'clothes' is the definition.
(threads is a kind of clothes)
'hardest to repair' is the wordplay.
'to repair' is an anagram indicator (letters are fixed or repaired).
'hardest' is an anagram of 'THREADS'.
(Other definitions for threads that I've seen before include "Slang for suits, or fibres" , "Strands for sewing" , "Spiral ridges of screw or thin lengths" , "Lengths of yarn" , "Clothes (slang)" .)