Equestrian in condition (5)
I believe the answer is:
rider
I believe this clue is a double definition.
'equestrian' is the first definition.
(I've seen this in another clue)
'condition' is the second definition.
(condition or specification added to a contract)
'in' acts as a link.
(Other definitions for rider that I've seen before include "One may be saddled" , "Horseman or addendum" , "supported by an Arab, perhaps" , "Contractual clause" , "Person in the saddle" .)