Rope — look — restraining donkey (5)
I believe the answer is:
lasso
'rope' is the definition.
(lassoing is a kind of roping)
'look restraining donkey' is the wordplay.
'look' becomes 'lo'.
'restraining' means one lot of letters goes inside another.
'donkey' becomes 'ass' (I've seen this before).
'lo' enclosing 'ass' is 'LASSO'.
(Other definitions for lasso that I've seen before include "Rope with noose for catching animals" , "Cowboy's noosed rope" , "use one for catching cattle" , "Rope for catching livestock" , "Looped rope used to catch cattle" .)