Working dog belonging to us (6)
I believe the answer is:
labour
'working' is the definition.
'dog belonging to us' is the wordplay.
'dog' becomes 'lab' (short for labrador).
'belonging to us' becomes 'our' (I've seen this before).
'lab'+'our'='LABOUR'
(Other definitions for labour that I've seen before include "See 1" , "Left - work" , "Workforce" , "Hard work, party" , "Men are never in this" .)