The odds on impostor's dog (8)
I believe the answer is:
springer
'dog' is the definition.
(I've seen this before)
'the odds on impostor's' is the wordplay.
'the odds' becomes 'sp' (betting abbreviation for starting price).
'on' is a charade indicator (letters next to each other).
'impostor' becomes 'ringer' (ringer is a kind of impostor).
'sp'+'ringer'='SPRINGER'
(Other definitions for springer that I've seen before include "Shown [AT CRUFTS]" , "A spaniel; stone in arch" , "Type of spaniel" , "Large spaniel with wavy silky coat" , "Breed of spaniel" .)