Left donkey attached to old rope (5)
I believe the answer is:
lasso
'rope' is the definition.
(lassoing is a kind of roping)
'left donkey attached to old' is the wordplay.
'left' becomes 'l' (common abbreviation).
'donkey' becomes 'ass' (I've seen this before).
'attached to' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'l'+'ass'+'o'='LASSO'
(Other definitions for lasso that I've seen before include "Rancher's rope" , "Nosed rope" , "Cowcatcher" , "Ling-nosed rope" , "Cowboy's noosed rope" .)