Green, not unusual (6)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'green' is the first definition.
('common' can be a synonym of 'green')
'not unusual' is the second definition.
(Other definitions for common that I've seen before include "Vulgar; shared", "Average, ordinary or usual", "Without special distinction, ordinary", "Ordinary, not refined", "Ten a penny", "Plentiful or vulgar", "General or of low degree", "Regular winter illness", "Ill-bred, not refined in manner" and "Ordinary; open land".)