Retired father and mother, say? That's obvious (8)
I believe the answer is:
apparent
'obvious' is the definition.
('apparent' can be similar in meaning to 'obvious')
'retired father and mother say?' is the wordplay.
'retired' shows that the letters should be reversed in order.
'father' becomes 'pa' (pa is a kind of father).
'and' means one lot of letters go next to another.
'mother say?' becomes 'parent' (mother is a kind of parent**).
'pa' back-to-front is 'ap'.
'ap'+'parent'='APPARENT'
'that's' is the link.
(Other definitions for apparent that I've seen before include "Conspicuous" , "readily understood" , "Readily perceived" , "Clearly visible" , "Evident or seeming to be" .)