New farmer's content to have fifty in field (5)
I believe the answer is:
realm
'field' is the definition.
(I know that realm is a type of field)
'new farmer's content to have fifty' is the wordplay.
'new' indicates an anagram.
'content' says to take the centre.
'to have' is an insertion indicator.
'fifty' becomes 'l' (Roman numeral).
The central letters of 'farmer' are 'arme'.
'arme' is an anagram of 'ream'.
'ream' going around 'l' is 'REALM'.
'in' acts as a link.
(Other definitions for realm that I've seen before include "Kingdom, domain" , "department" , "A Nation" , "Province" , "Kingdom; field of interest" .)