Dad upset mum? That’s obvious! (8)
I believe the answer is:
apparent
'obvious' is the definition.
(similar in meaning)
'dad upset mum?' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'upset' says the letters should be written backwards.
'mum?' becomes 'parent' (mum is a kind of parent).
'pa' backwards is 'ap'.
'ap'+'parent'='APPARENT'
'that's' is the link.
(Other definitions for apparent that I've seen before include "Pane part may be quite obvious" , "as such an heir" , "Obvious; seemingly true" , "Seeming true; obvious" , "readily understood" .)