Dog working sheep (6)
I believe the answer is:
mutton
'sheep' is the definition.
(I've seen this before)
'dog working' is the wordplay.
'dog' becomes 'mutt' (mutt is a kind of dog).
'working' becomes 'on'.
'mutt'+'on'='MUTTON'
(Other definitions for mutton that I've seen before include "Type of meat" , "can be eaten" , "Sheep's flesh" , "Meat that may dress as lamb" , "Sheep's meat" .)