He pesters Hill, the advisor (9)
I believe the answer is:
tormentor
'he pesters' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'hill the advisor' is the wordplay.
'hill' becomes 'tor' (tor is a kind of hill).
'the advisor' becomes 'mentor' (I am not sure about this - if you are sure you should believe this answer much more).
'tor'+'mentor'='TORMENTOR'
Can you help me to learn more?
(Other definitions for tormentor that I've seen before include "One to harass" , "One giving great pain or annoyance" , "Inflictor of pain or anguish" , "One harassing" , "One harrying" .)