Green wall up protecting revolutionary army (8)
I believe the answer is:
immature
'green' is the definition.
('immature' can be a synonym of 'green')
'wall up protecting revolutionary army' is the wordplay.
'wall up' becomes 'immure' (I've seen this before).
'protecting' means one lot of letters goes inside another.
'revolutionary' says the letters should be written in reverse.
'army' becomes 'ta' (abbreviation for Territorial Army).
'ta' back-to-front is 'at'.
'immure' placed around 'at' is 'IMMATURE'.
(Other definitions for immature that I've seen before include "Undeveloped - unripe" , "Underdeveloped - childish" , "Not yet adult or fully developed" , "Young" , "Not yet fully grown or developed" .)