Less common (5)
I believe the answer is:
rarer
'less common' is the definition.
('rare' can be the opposite of 'common' so perhaps 'rarer' is reasonable for 'less common')
This is all the clue.
(Other definitions for rarer that I've seen before include "More unusual" , "steak may be wanted thus?" , "More scarce or uncommon" , "Comparatively sparse" , "(Of meat) less cooked" .)