Exhaust dog with constant exercise (8)
I believe the answer is:
tailpipe
'exhaust' is the definition.
(I've seen this before)
'dog with constant exercise' is the wordplay.
'dog' becomes 'tail' (synonyms).
'with' says to put letters next to each other.
'constant' becomes 'pi' (mathematics: i.tl).
'exercise' becomes 'pe' (Physical Education).
'tail'+'pi'+'pe'='TAILPIPE'
(Other definitions for tailpipe that I've seen before include "Rear section of exhaust" , "Car exhaust (US)" , "End to an exhaust system" .)