Equestrian's condition (5)
I believe the answer is:
rider
This clue is a double definition.
'equestrian's' is the first definition.
(thesaurus)
'condition' is the second definition.
(I've seen this before)
(Other definitions for rider that I've seen before include "Condition added to something already agreed" , "H. - Haggard, English novelist (She, etc)" , "Possibly cavalier" , "Passenger" , "Supplementary verdict brought in by jury" .)