Leslie endlessly unravels the thread (5)
I believe the answer is:
lisle
'the thread' is the definition.
(lisle is a kind of thread)
'leslie endlessly unravels' is the wordplay.
'endlessly' means to remove the last letter.
'unravels' is an anagram indicator (I've seen 'unravelling' mean this).
'leslie' with its final letter taken off is 'lesli'.
'lesli' anagrammed gives 'LISLE'.
(Other definitions for lisle that I've seen before include "Strong cotton fabric" , "Strong cotton material" , "Twisted yarn" , "Strong twisted cotton thread for the strange Ellis" , "Fine smooth cotton thread for stockings" .)