He's making himself at home in the dog pound (7)
I believe the answer is:
settler
'he's making himself at home' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in the dog pound' is the wordplay.
'in the' indicates putting letters inside.
'dog' becomes 'setter' (setter is a type of dog).
'pound' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'setter' placed around 'l' is 'SETTLER'.
Can you help me to learn more?
(Other definitions for settler that I've seen before include "Coloniser - arbitrator" , "Early colonist" , "new inhabitant" , "One taking up residence in a new area" , "Colonist, pioneer" .)