Most unusual (6)
'most unusual' is the definition.
(rare can mean unusual or odd)
This is the entire clue.
(Other definitions for rarest that I've seen before include "Most scarce or least common" , "Most scarce or uncommon" , "Most scarce and probably precious" , "least done" , "Least common, most unusual" .)
'most unusual' is the definition.
('odd' can be similar in meaning to 'unusual')
This is all the clue.
(Other definitions for oddest that I've seen before include "Most rum" , "Queerest" , "Strangest" , "Most peculiar or strange" , "Most strange or curious" .)