Plain father's back with mother (8)
I believe the answer is:
apparent
'plain' is the definition.
('apparent' can be a synonym of 'plain')
'father's back with mother' is the wordplay.
'father' becomes 'pa' (pa is a kind of father).
'back' is a reversal indicator.
'with' says to put letters next to each other.
'mother' becomes 'parent' (I've seen this in other clues**).
'pa' reversed gives 'ap'.
'ap'+'parent'='APPARENT'
(Other definitions for apparent that I've seen before include "Pane part may be quite obvious" , "Obvious, clearly understood" , "Evident or seeming to be" , "Clearly visible" , "Seeming to be but not necessarily so" .)