Clause for an equestrian (5)
I believe the answer is:
rider
I believe this clue is a double definition.
'clause' is the first definition.
(rider is a kind of clause)
'an equestrian' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for rider that I've seen before include "qualification" , "Jury's recommendation" , "Person on horseback" , "Horseman or supplementary jury verdict" , "Jockey" .)