Green pistachio nuts cracked by Edward (15)
I believe the answer is:
unsophisticated
'green' is the definition.
(I've seen this before)
'pistachio nuts cracked by edward' is the wordplay.
'cracked' indicates an anagram.
'by' means one lot of letters go next to another.
'edward' becomes 'Ed' (short for Edward).
'pistachio'+'nuts'='pistachionuts'
'pistachionuts' anagrammed gives 'unsophisticat'.
'unsophisticat'+'ed'='UNSOPHISTICATED'
(Other definitions for unsophisticated that I've seen before include "Lacking experience" , "green" , "crude" , "Simple" , "straightforward" .)