Irreligious father with a horse to back (5)
I believe the answer is:
pagan
'irreligious' is the definition.
(similar in meaning)
'father with a horse to back' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'with' is a charade indicator (letters next to each other).
'a horse' becomes 'nag' (nag is a kind of horse).
'to back' says the letters should be written in reverse.
'nag' backwards is 'gan'.
'pa'+'gan'='PAGAN'
(Other definitions for pagan that I've seen before include "One outside the main religions" , "Non-mainstream religious believer" , "Heathen; non-believer" , "One who follows a polytheistic or pre-Christian religion, but not a Jew" , "Unbeliever" .)