Nun's remains in nameless grave (6)
I believe the answer is:
sister
'nun's' is the definition.
(a nun might be called sister)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sister that I've seen before include "Nurse, nun and sibling" , "of the same origin" , "Resist about the sibling" , "Relative in hospital?" , "trade union member?" .)