A harvest shortened field (4)
I believe the answer is:
area
'field' is the definition.
(I've seen this before)
'a harvest shortened' is the wordplay.
'harvest' becomes 'reap' ('reap' can be a synonym of 'harvest').
'shortened' means to remove the last letter.
'reap' with its final letter taken off is 'rea'.
'a'+'rea'='AREA'
(Other definitions for area that I've seen before include "environs" , "Extent of a two-dimensional surface" , "Patch" , "Region, zone" , "Region, space" .)