Official beginning to mow field (5)
I believe the answer is:
realm
'field' is the definition.
(I know that realm is a type of field)
'official beginning to mow' is the wordplay.
'official' becomes 'real'.
'beginning to' suggests taking the first letters.
The first letter of 'mow' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Area of knowledge under discussion" , "Monarchy" , "Area of influence" , "Country" , "Empire - state" .)