One working with horses for firm on island (9)
I believe the answer is:
stableman
'one working with horses' is the definition.
'firm on island' is the wordplay.
'firm' becomes 'stable' ('stable' can be similar in meaning to 'firm'**).
'on' is a charade indicator (letters next to each other).
'island' becomes 'man' (Man is an example).
'stable'+'man'='STABLEMAN'
'for' is the link.
(Other definitions for stableman that I've seen before include "Maybe horse trainer" , "Ostler, groom etc" .)