Reunite it with new string (5)
I believe the answer is:
twine
'string' is the definition.
(I know that string can be written as twine)
'reunite it with new' is the wordplay.
'reunite' is an anagram indicator (I've seen 'reuniting' mean this (letters are put together again in a new way)).
'with' says to put letters next to each other.
'it' put after 'new' is 'newit'.
'newit' is an anagram of 'TWINE'.
(Other definitions for twine that I've seen before include "String, cord" , "Interweave" , "Coil" , "Thin cord" , "Stout string" .)