Spheres in which one's little brothers circulate (4)
I believe the answer is:
orbs
'spheres' is the definition.
(orb is a kind of sphere)
'one's little brothers circulate' is the wordplay.
'one' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'little' means to remove the last letter (the word is not quite complete).
'brothers' becomes 'bros'.
'circulate' says the letters should be written in reverse (I've seen 'circulating' mean this).
'bros' with its final letter taken off is 'bro'.
's'+'bro'='sbro'
'sbro' back-to-front is 'ORBS'.
'in which' acts as a link.
Can you help me to learn more?
(Other definitions for orbs that I've seen before include "Spherical objects" , "Poetic eyes, bros" , "circles" , "Jewelled spheres" , "Ball shapes" .)