Starts to observe radiation belts surrounding celestial bodies (4)
I believe the answer is:
orbs
'celestial bodies' is the definition.
(I've seen this in another clue)
'starts to observe radiation belts surrounding' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'observe radiation belts surrounding' is 'ORBS'.
(Other definitions for orbs that I've seen before include "Eyes - globes" , "Nices eyes, bros" , "Ball shapes" , "Round objects" , "Poetic eyes, bros" .)