A horses nay (7)
I believe the answer is:
refusal
'nay' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for refusal that I've seen before include "fault in the show ring" , "Dissent" , "Rejection of an offer" , "Chance to decline" , "Possibly first option to buy" .)