Back dad and mum as such an heir? (8)
I believe the answer is:
apparent
'as such an heir?' is the definition.
(I've seen this before)
'back dad and mum' is the wordplay.
'back' is a reversal indicator.
'dad' becomes 'pa' (both can mean one's father).
'and' says to put letters next to each other.
'mum' becomes 'parent'.
'pa' back-to-front is 'ap'.
'ap'+'parent'='APPARENT'
(Other definitions for apparent that I've seen before include "Obvious, evident" , "See 1" , "Noticeable" , "as such an heir" , "Obvious; seemingly true" .)