River and tree bordering area in field (5)
I believe the answer is:
realm
'field' is the definition.
(I know that realm is a type of field)
'river and tree bordering area' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'and' is a charade indicator (letters next to each other).
'tree' becomes 'elm' (elm is a kind of tree).
'bordering' means one lot of letters goes inside another.
'area' becomes 'a' (maths abbreviation).
'elm' going around 'a' is 'ealm'.
'r'+'ealm'='REALM'
'in' is the link.
(Other definitions for realm that I've seen before include "region" , "Kingdom, or field of activity" , "department" , "Kingdom; field of interest" , "Area of influence" .)