Strong tightly twisted cotton thread (5)
I believe the answer is:
lisle
'strong tightly twisted cotton thread' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lisle that I've seen before include "Strong cotton material" , "Cotton stocking thread" , "Strong tightly twisted cotton thread" , "Fine cotton thread used in stockings" , "Strong twisted cotton thread for the strange Ellis" .)