Ostensibly round balls, spheres primarily? (4)

I believe the answer is:

orbs

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'ostensibly round balls spheres primarily?' is the definition.
I can't tell whether this definition defines the answer.

'ostensibly round balls spheres primarily?' is the wordplay.
'primarily?' suggests taking the first letters (I've seen 'primarily' mean this).
The initial letters of 'ostensibly round balls spheres' is 'ORBS'.

Can you help me to learn more?

(Other definitions for orbs that I've seen before include "sight" , "circles" , "Eyes - globes" , "Spherical shapes" , "Eyeballs" .)

I've seen this clue in The Times.

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