River near 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 near tree bordering area' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'near' is a charade indicator (letters next to each other).
'tree' becomes 'elm' (elm tree is a kind of tree).
'bordering' means one lot of letters goes inside another.
'area' becomes 'a' (maths abbreviation).
'elm' enclosing 'a' is 'ealm'.
'r'+'ealm'='REALM'
'in' acts as a link.
(Other definitions for realm that I've seen before include "Royal domain" , "where ruler holds sway?" , "Knowledge base" , "Dominion" , "Kingdom; field of interest" .)