Are at the far end, which is not customary (4)
I believe the answer is:
rare
'is not customary' is the definition.
I can't tell whether this defines the answer.
'are at the far end' is the wordplay.
'at' says to put letters next to each other (I've seen this in other clues).
'end' suggests the final letters.
The last letter of 'far' is 'r'.
'are' put after 'r' is 'RARE'.
'which' acts as a link.
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" .)