Are at the far end, which is not customary (4)
I believe the answer is:
rare
'end which is not customary' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?
'are at the far' is the wordplay.
'at' means one lot of letters go next to another (I've seen this in other clues).
'the far' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'are' put after 'r' is 'RARE'.
Can you help me to learn more?
(Other definitions for rare that I've seen before include "Singular, infrequent" , "Occurring infrequently" , "Not found often" , "thin on the ground" , "Exceptional" .)