Horseman's corollary (5)
I believe the answer is:
rider
This could be a double definition.
'horseman's' is the first definition.
(someone riding a horse)
'corollary' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "One on horse or bike" , "one making journey" , "Condition added to a legal document" , "in the dressing room before the show?" , "One may be saddled" .)