Thread that only breaks when the end's broken off (5)
I believe the answer is:
nylon
'thread' is the definition.
(thesaurus)
'only breaks when the end's broken off' is the wordplay.
I cannot really understand how this works, but
an anagram of 'only' is 'nylo' which is within the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'that' is the link.
Can you help me to learn more?
(Other definitions for nylon that I've seen before include "where ladder can appear?" , "may be damaged by running" , "The first synthetic fibre" , "Synthetic polymer" , "Stocking; polymer" .)