Horse always gets fodder (3)
'fodder' is the definition.
(hay is a kind of fodder)
'horse always' is the wordplay.
'horse' becomes 'h' (synonyms).
'always' becomes 'ay' (archaic term for 'always').
'gets' acts as a link.
(Other definitions for hay that I've seen before include "Animal fodder", "It is to be made while the sun shines", "Forage", "basic bedding", "Meadow crop".)