Least common (6)
'least common' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oddest that I've seen before include "Queerest" , "Most strange or curious" , "Most peculiar or uneven" , "Most unusual or uneven" , "Most eccentric or peculiar" .)
'least common' is the definition.
('rare' is the opposite of 'common' so 'rarest' should be a synonym for 'least common')
This is the entire clue.
(Other definitions for rarest that I've seen before include "Most scarce and probably precious" , "Most scarce or least common" , "Least common, most unusual" , "Most scarce or uncommon" , "fine to maximum" .)