Dog riding sheep (6)
I believe the answer is:
mutton
'sheep' is the definition.
(I've seen this before)
'dog riding' is the wordplay.
'dog' becomes 'mutt' (mutt is a kind of dog).
'riding' becomes 'on' (I've seen this in other clues).
'mutt'+'on'='MUTTON'
(Other definitions for mutton that I've seen before include "Meat that may dress as lamb" , "can be eaten" , "Dead, we are told" , "Sheep as food" , "Flesh of sheep" .)