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' (term for an old horse).
'to back' shows that the letters should be reversed in order.
'nag' reversed gives '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" .)