Origin of cheap green crop (4)
I believe the answer is:
craw
'crop' is the definition.
('craw' can be a synonym of 'crop')
'origin of cheap green' is the wordplay.
'origin of' says to take the initial letters.
'green' becomes 'raw' ('raw' can be a synonym of 'green').
The first letter of 'cheap' is 'c'.
'c'+'raw'='CRAW'
(Other definitions for craw that I've seen before include "Bird's crop" , "Crop of a bird" , "Gizzard" , "Bird's pouch for storing food" , "Stomach of an animal" .)