Furthest back (8)
'furthest back' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rearmost that I've seen before include "Fully behind" , "last" , "More rats (anag)" , "farthest from the front" , "Furthest back" .)
(Other definitions for hindmost that I've seen before include "after all the others" , "One at the back (devil take him!)" , "The one who is furthest behind - devil take him" , "Last" , "Like a tail-ender" .)