Green, coup d'état his sin, taken out (15)
I believe the answer is:
unsophisticated
'green' is the definition.
(I've seen this before)
'coup d'etat his sin taken out' is the wordplay.
'taken out' indicates an anagram.
'coup'+'detat'+'his'+'sin'='coupdetathissin'
'coupdetathissin' anagrammed gives 'UNSOPHISTICATED'.
(Other definitions for unsophisticated that I've seen before include "crude" , "straightforward" , "Lacking experience" , "Simple" , "green" .)