Bones rot and decay in interment, initially (5)
I believe the answer is:
radii
'bones' is the definition.
(radius is a kind of bone)
'rot and decay in interment initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'rot and decay in interment' is 'RADII'.
(Other definitions for radii that I've seen before include "Straight lines from centre of a circle" , "Geometrical lines" , "Bones of the human forearm" , "Lines from a circle's centre to the circumference" , "Lines from the centre to the circumference of a circle" .)