Hardest shape for clothes (7)
I believe the answer is:
threads
'clothes' is the definition.
(threads is a kind of clothes)
'hardest shape' is the wordplay.
'shape' is an anagram indicator.
'hardest' anagrammed gives 'THREADS'.
'for' acts as a link.
(Other definitions for threads that I've seen before include "Lengths of yarn" , "Strands of yarn" , "lines" , "Hardest (anag.)" , "Slang for suits, or fibres" .)