Crop circles essentially not manufactured (4)
I believe the answer is:
craw
'crop' is the definition.
(synonyms)
'circles essentially not manufactured' is the wordplay.
'essentially' says to take the centre.
'not manufactured' becomes 'raw' (I've seen this before).
The central letter of 'circles' is 'c'.
'c'+'raw'='CRAW'
(Other definitions for craw that I've seen before include "throat" , "Bird's pouch for storing food" , "Stomach of an animal" , "Bird's crop" , "Gizzard" .)