Reddest inside (6)
I believe the answer is:
rarest
'reddest inside' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rarest that I've seen before include "Most scarce or least common" , "Least common, most unusual" , "least done" , "Most scarce or uncommon" , "fine to maximum" .)