Sort of thread Leslie endlessly tangles (5)
I believe the answer is:
lisle
'sort of thread' is the definition.
'leslie endlessly tangles' is the wordplay.
'endlessly' means to remove the last letter.
'tangles' indicates an anagram (I've seen 'tangle' mean this).
'leslie' with its final letter taken away is 'lesli'.
'lesli' with letters rearranged gives 'LISLE'.
(Other definitions for lisle that I've seen before include "Strong cotton material" , "Strong twisted cotton thread for the strange Ellis" , "Strong tightly twisted cotton thread" , "Fine cotton thread used in stockings" , "Cotton stocking thread" .)