Threads appear to change length (7)
I believe the answer is:
apparel
'threads' is the definition.
(I've seen this in another clue)
'appear to change length' is the wordplay.
'to change' is an anagram indicator.
'length' becomes 'l' (abbreviation in geometry).
'appear' is an anagram of 'appare'.
'appare'+'l'='APPAREL'
(Other definitions for apparel that I've seen before include "it's generally put on!" , "Formal word for clothing" , "Clothes - garments" , "what's in the press?" , "Raiment" .)