One who guards a dog on a hill (7)
I believe the answer is:
curator
'one who guards a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'dog on a hill' is the wordplay.
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'on' says to put letters next to each other.
'hill' becomes 'tor' (tor is a kind of hill).
'cur'+'a'+'tor'='CURATOR'
Can you help me to learn more?
(Other definitions for curator that I've seen before include "Museum's administrative head" , "Person in charge of a museum" , "museum boss" , "Guardian" , "Artistic director" .)