The rope's new, unbound, lacking length (5)
I believe the answer is:
noose
'the rope's' is the definition.
(noose is a kind of rope)
'new unbound lacking length' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'unbound' becomes 'loose' (similar in meaning).
'lacking' indicates named letters should be taken away.
'length' becomes 'l' (abbreviation in geometry).
'loose' with 'l' taken away is 'oose'.
'n'+'oose'='NOOSE'
(Other definitions for noose that I've seen before include "Loop at the end of a rope" , "old execution method" , "A loop for the hangman" , "Loop in rope that tightens when pulled" , "Hanging material" .)