Masters the conversion of hectares (8)
I believe the answer is:
teachers
'masters' is the definition.
(as in schoolmasters)
'conversion of hectares' is the wordplay.
'conversion of' indicates an anagram (I've seen 'conversion' mean this).
'hectares' is an anagram of 'TEACHERS'.
'the' is the link.
(Other definitions for teachers that I've seen before include "They impart knowledge" , "Lecturers" , "beaks?" , "Coaches" , "Instuctors" .)