Most uncommon (6)
I believe the answer is:
rarest
'most uncommon' is the definition.
('rare' can be similar in meaning to 'uncommon')
This is all the clue.
(Other definitions for rarest that I've seen before include "Least common, most unusual" , "most excellent" , "Most scarce or least common" , "fine to maximum" , "Most scarce or infrequent" .)