Not as common (5)
I believe the answer is:
rarer
'not as common' is the definition.
('rare' can be the opposite of 'common' so perhaps 'rarer' is reasonable for 'not as common')
This is all the clue.
(Other definitions for rarer that I've seen before include "More scarce or uncommon" , "Thinner" , "Comparatively sparse" , "even less common" , "More unusual" .)