Cornfield intruder (4)
I believe the answer is:
crow
'cornfield intruder' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for crow that I've seen before include "Ugly old woman" , "Express gleeful triumph" , "Black bird; brag" , "Blow one's own trumpet" , "Boast gloatingly, American Indian" .)