Father has no hesitation accepting one belief (5)
I believe the answer is:
faith
'belief' is the definition.
(faith is a kind of belief)
'father has no hesitation accepting one' is the wordplay.
'has no' suggests deleting specific letters.
'hesitation' becomes 'er'.
'accepting' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'father' with 'er' removed is 'fath'.
'fath' placed around 'i' is 'FAITH'.
(Other definitions for faith that I've seen before include "religion" , "'..... will move mountains'" , "Unshakeable belief" , "Religious belief" , "A woman" .)