A breather after run that's most unusual (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'a breather after run' is the wordplay.
'breather' becomes 'rest' (I've seen this before).
'after' says to put letters next to each other.
'run' becomes 'r' (cricket abbreviation).
'a'+'rest'='arest'
'arest' after 'r' is 'RAREST'.
'that's' acts as a link.
(Other definitions for rarest that I've seen before include "Least common, most unusual" , "fine to maximum" , "Sartre (anag) -- most uncommon" , "least done" , "Most scarce or infrequent" .)