Initially rotating arms does involve individual bones (5)
I believe the answer is:
radii
'bones' is the definition.
(radius is a kind of bone)
'initially rotating arms does involve individual' is the wordplay.
'initially' suggests taking the first letters.
'individual' becomes 'i'.
The initial letters of 'rotating arms does involve' is 'radi'.
'radi'+'i'='RADII'
(Other definitions for radii that I've seen before include "They meet in the middle of a circle" , "Bones of the human forearm" , "Forearm bones: spokes" , "more than one spoke" , "lines from a central part" .)