Over him is the supervisor (4)
I believe the answer is:
seer
'over him is the supervisor' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'him is the supervisor' is the wordplay.
'him' becomes 'See'.
'is the supervisor' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'see'+'r'='SEER'
Can you help me to learn more?
(Other definitions for seer that I've seen before include "One fortelling future" , "One who knows the future" , "Person of vision" , "Prophet or visionary" , "One having visions of the future" .)