He's in charge of one farm possibly (7)
I believe the answer is:
foreman
'he's in charge' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one farm possibly' is the wordplay.
'possibly' indicates an anagram.
'one'+'farm'='onefarm'
'onefarm' is an anagram of 'FOREMAN'.
'of' is the link.
Can you help me to learn more?
(Other definitions for foreman that I've seen before include "Charge hand" , "Gaffer, overseer" , "Gaffer, boss" , "Fellow in charge" , "Male supervisor" .)