One working with horses for firm on island (9)
I believe the answer is:
Here is my best explanation:
'one working with horses' is the definition.
'firm on island' is the subsidiary indication.
'firm' becomes 'stable' ('stable' can be similar in meaning to 'firm'**).
'on' means one lot of letters go next to another.
'island' becomes 'man' (Man is an example).
'for' is the link.
(Other definitions for stableman that I've seen before include "Maybe horse trainer", "Groom" and "Ostler, groom etc".)