Jockey in condition? (5)
I believe the answer is:
rider
I believe this clue is a double definition.
'jockey' is the first definition.
(horse rider)
'condition?' is the second definition.
(condition or specification added to a contract)
'in' is the link.
(Other definitions for rider that I've seen before include "Condition - under which a horse races" , "Clause appended to a legislative bill" , "Condition added to a legislative bill" , "Proviso in a legal document" , "Equestrian added by a jury?" .)