Supervisor of one farm (7)
I believe the answer is:
foreman
'supervisor' is the definition.
(I know that foreman is a type of supervisor)
'of one farm' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'one'+'farm'='onefarm'
'onefarm' is an anagram of 'FOREMAN'.
(Other definitions for foreman that I've seen before include "Gaffer, overseer" , "spokesman" , "Supervising worker" , "Charge hand" , "Fellow in charge" .)