Loop with running knot (5)
I believe the answer is:
noose
'loop with running knot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for noose that I've seen before include "Hangman's or cowboy's loop" , "Loop - snare" , "Loop at the end of a rope" , "Loop formed by means of a slip knot" , "Hanging loop in rope" .)