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