A harvest unfinished in field (4)
I believe the answer is:
area
'field' is the definition.
(I've seen this before)
'a harvest unfinished' is the wordplay.
'harvest' becomes 'reap' ('reap' can be a synonym of 'harvest').
'unfinished' means to remove the last letter.
'reap' with its last letter taken away is 'rea'.
'a'+'rea'='AREA'
'in' acts as a link.
(Other definitions for area that I've seen before include "Expanse or neighbourhood" , "Geographical region" , "Field -- expanse" , "part of town" , "Region, zone" .)