Equestrian clause (5)
I believe the answer is:
rider
I believe this clue is a double definition.
'equestrian' is the first definition.
(I've seen this before)
'clause' is the second definition.
(I know that rider is a type of clause)
(Other definitions for rider that I've seen before include "Clause added to a legal document" , "one making journey" , "Equestrian added by a jury?" , "recommendation for mercy?" , "Proviso in a legal document" .)