A body on motorway is dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(mastiff is a kind of dog)
'a body on motorway' is the wordplay.
'body' becomes 'stiff' (stiff is a kind of body).
'on' says to put letters next to each other.
'motorway' becomes 'm'.
'a'+'stiff'='astiff'
'astiff' after 'm' is 'MASTIFF'.
'is' acts as a link.
(Other definitions for mastiff that I've seen before include "Large short-haired dog breed" , "Hound" , "Powerful breed of watchdog" , "Large, powerful, short-haired dog" , "Big dog" .)