Father’s on back seat — one vacated by dog (8)
I believe the answer is:
papillon
'dog' is the definition.
(papillon is a kind of dog)
'father's on back seat one vacated' is the wordplay.
I cannot quite understand how this works, but
'on' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'father' could be 'pa' (pa can mean dad or father) and 'pa' is present in the answer.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for papillon that I've seen before include "pet dog" , "Dog breed - Steve McQueen 1973 role" .)