Go unusually green at the end of May (6)
I believe the answer is:
energy
'go' is the definition.
(I've seen this before)
'unusually green at the end of may' is the wordplay.
'unusually' is an anagram indicator.
'at the end of' indicates one should take the final letters.
The last letter of 'may' is 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Vitality, get-up-and-go" , "Zest" , "Gusto" , "Go, zip" , "Vigour, vim, vitality" .)