Keep moving in part of person's sphere of vision (6)
I believe the answer is:
retina
'person's sphere of vision' is the definition.
I can't tell whether this defines the answer.
'keep moving in part' is the wordplay.
'keep moving' becomes 'reta' (I can't explain this - if you can you should believe this answer much more).
'part' means one lot of letters goes inside another (the inserted letters split open or part the word).
'reta' going around 'in' is 'RETINA'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for retina that I've seen before include "Membrane covering the back of the eyeball" , "Ocular membrane" , "part of the water pump" , "something that helps conjure up image" , "Sensitive part of eye" .)