One father or two fathers (4)
I believe the answer is:
papa
'one father' is the definition.
(papa is a kind of father)
'two fathers' is the wordplay.
'two fathers' can mean two replacements for 'father'.
'father' becomes 'pa' (pa can mean dad or father).
'father' becomes 'pa' (pa can mean dad or father).
'pa'+'pa'='PAPA'
'or' is the link.
(Other definitions for papa that I've seen before include "The old man" , "Pop" , "one after Oscar" , "Dad" , "Father; letter P" .)