He looks after a dog on a hill (7)
I believe the answer is:
curator
'he looks' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a dog on a hill' is the wordplay.
'a dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'hill' becomes 'tor' (tor is a kind of hill).
'cur'+'a'+'tor'='CURATOR'
'after' acts as a link.
Can you help me to learn more?
(Other definitions for curator that I've seen before include "Custodian of a collection" , "Museum keeper" , "Museum's administrative head" , "Museum custodian" , "museum boss" .)