Stable with one working horse (8)
I believe the answer is:
Here is my best explanation:
'horse' is the definition.
(I've seen this before)
'stable with one working' is the subsidiary indication.
'stable' becomes 'stall' (I've seen this in other clues).
'with' means one lot of letters go next to another.
'one' becomes 'i' (roman numeral).
'working' becomes 'on'.
(Other definitions for stallion that I've seen before include "Male steed", "Male horse lost in atolls", "Male horse, often used for breeding", "Mount", "Uncastrated adult male horse", "Male horse kept for breeding", "stud", "Mare's mate", "Stud" and "Horse kept for breeding".)