Father raised horse - he's a heathen (5)
I believe the answer is:
pagan
'heathen' is the definition.
(both can mean an irreligious person)
'father raised horse' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'raised' says the letters should be written backwards (going up in a down clue).
'horse' becomes 'nag' (term for an old horse).
'nag' reversed gives 'gan'.
'pa'+'gan'='PAGAN'
'he's a' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pagan that I've seen before include "Heathen; non-believer" , "One who does not acknowledge a god" , "One who does not acknowledge your god(s)" , "Non-mainstream religious believer" , "Polytheist" .)