Right; a remainder occurring furthest apart (6)
I believe the answer is:
rarest
'occurring furthest apart' is the definition.
I can't tell whether this definition defines the answer.
'right a remainder' is the wordplay.
I cannot really understand how this works, but
'right' could be 'ar' (I've seen this in other clues) and 'ar' is located in the answer.
'remainder' could be 'rest' (synonyms) and 'rest' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rarest that I've seen before include "Sartre (anag) -- most uncommon" , "most excellent" , "Most scarce or infrequent" , "fine to maximum" , "Most scarce and probably precious" .)