Unproductive field, northern (4)
I believe the answer is:
lean
'unproductive' is the definition.
(I've seen this before)
'field northern' is the wordplay.
'field' becomes 'lea' (I've seen this before**).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'lea'+'n'='LEAN'
(Other definitions for lean that I've seen before include "Of meat, with little or no fat" , "Incline, bend" , "Unproductive" , "With no fat; slope" , "Without excess flesh" .)