Well done by a horse and West! (6)
I believe the answer is:
stewed
'well' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'a horse and west' is the wordplay.
I cannot quite see how this works, but
an anagram of 'west' is 'stew' which is located in the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'done by' is the link.
Can you help me to learn more?
(Other definitions for stewed that I've seen before include "Drunk; cooked in pot" , "(Meat) simmered; (tea) over-infused" , "canned" , "mortal" , "Of tea, left brewing too long" .)