Field with northern slope (4)
I believe the answer is:
lean
'slope' is the definition.
(I know that slope is a more specific form of lean)
'field with northern' is the wordplay.
'field' becomes 'lea' (lea can mean a meadow or field of grass).
'with' is a charade indicator (letters next to each other).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'lea'+'n'='LEAN'
(Other definitions for lean that I've seen before include "Tend towards being thin" , "without success" , "bank" , "Slender" , "Unprofitable" .)