Horse's one of five in eastern Spain (6)
I believe the answer is:
equine
'horse's' is the definition.
(thesaurus)
'one of five in eastern spain' is the wordplay.
'one of five' becomes 'quin' (I've seen this before).
'in' is an insertion indicator.
'eastern' becomes 'e'.
'spain' becomes 'e' (abbreviation for Espana).
'e'+'e'='ee'
'quin' placed into 'ee' is 'EQUINE'.
(Other definitions for equine that I've seen before include "like knight?" , "Relating to horses" , "Horse or donkey, perhaps" , "Like a horse" , "Horse-like" .)