Bones of a philosopher mostly rest in peace (6)
I believe the answer is:
humeri
'bones' is the definition.
(I've seen this before)
'a philosopher mostly rest in peace' is the wordplay.
'a philosopher' becomes 'hume' (I've seen this in other clues).
'mostly' means to remove the last letter.
'rest in peace' becomes 'rip' (I've seen this before).
'rip' with its last letter taken off is 'ri'.
'hume'+'ri'='HUMERI'
'of' is the link.
(Other definitions for humeri that I've seen before include "Bones from shoulder to elbow, in Latin plural" , "Components of arms" , "Upper-arm bones" , "Bones form shoulder to elbow" , "Bones in upper arm from shoulder to elbow" .)