Jacob’s father lives with a retired accountant (5)
I believe the answer is:
isaac
'jacob's father' is the definition.
(biblical figure)
'lives with a retired accountant' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'with' means one lot of letters go next to another.
'retired' says the letters should be written backwards.
'accountant' becomes 'CA' (abbreviation for chartered accountant).
'ca' back-to-front is 'ac'.
'is'+'a'+'ac'='ISAAC'
(Other definitions for isaac that I've seen before include "- Asimov, sci-fi writer" , "Abraham's son" , "Man's name" , "-- Asimov, sci-fi author" , "Son of Abraham, offered as a sacrifice to God" .)