Strong thread or string (5)
I believe the answer is:
twine
I believe this is a double definition.
'strong thread' is the first definition.
(I've seen this in another clue)
'string' is the second definition.
(I know that string can be written as twine)
(Other definitions for twine that I've seen before include "Twisted cord" , "String of twisted fibres" , "Tough string" , "Stout string" , "twist" .)