Not entirely green, a capital city (4)
I believe the answer is:
lima
'capital city' is the definition.
(capital of Peru)
'not entirely green a' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'green' becomes 'lime' (lime is a shade of green).
'lime' with its last letter removed is 'lim'.
'lim'+'a'='LIMA'
(Other definitions for lima that I've seen before include "Mail it to Peruvian city" , "South American capital" , "Mail to Peru's capital" , "Peruvian capital" , "Kind of bean" .)