Before end of May, English cornfield unusually green (3-8)
I believe the answer is:
eco-friendly
'green' is the definition.
(abbreviation for ecological)
'before end of may english cornfield unusually' is the subsidiary indication.
'before' is a charade indicator (letters next to each other).
'end of' suggests the final letters.
'english' becomes 'e' (abbreviation).
'unusually' indicates anagramming the letters.
The last letter of 'may' is 'y'.
'cornfield' is an anagram of 'cofriendl'.
'e'+'cofriendl'='ecofriendl'
'y' after 'ecofriendl' is 'ecofriendly'.
(Other definitions for eco-friendly that I've seen before include "Not harmful to the environment" , "In harmony with the environment" .)