Green garnet, possibly? (6)
I believe the answer is:
gretna
'green' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'garnet possibly?' is the wordplay.
'possibly?' indicates an anagram (I've seen 'possibly' mean this).
'garnet' is an anagram of 'GRETNA'.
Can you help me to learn more?
(Other definitions for gretna that I've seen before include "border village" , "... Green, for elopers" , "Green for eloping couples" , "Scottish town" .)