Stop one working for stud (8)
I believe the answer is:
Here is my best explanation:
'stud' is the definition.
'stop one working' is the subsidiary indication.
'stop' becomes 'stall' (stalling is a kind of stopping**).
'one' becomes 'i' (roman numeral).
'working' becomes 'on'.
'for' is the link.
(Other definitions for stallion that I've seen before include "Adult male horse", "Mare's mate", "Uncastrated adult male horse", "Male steed", "stud", "Horse kept for breeding", "Steed", "Male horse, often used for breeding", "Male horse lost in atolls" and "Stud".)