Left behind old rope (5)
I believe the answer is:
'rope' is the definition.
(I know that rope can be written as lasso)
'left behind old' is the wordplay.
I cannot really understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'behind' could be 'ass' (both can mean one's back side) and 'ass' is found in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lasso that I've seen before include "Rancher's rope" , "Cow catcher" , "Rope for throwing" , "Cattle-catching rope" , "Long noosed rope used to catch animals" .)