Sphere of some junior bookkeeper (3)
I believe the answer is:
orb
'sphere' is the definition.
(I know that orb is a type of sphere)
'some junior bookkeeper' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ORB' is hidden in the letters of 'junior bookkeeper'.
'of' acts as a link.
(Other definitions for orb that I've seen before include "Celestial body; sphere" , "Spherical symbol of monarch's power" , "Poetically eye" , "Item of insignia" , "component of royal regalia" .)