Nimbus with a long centre (4)

I believe the answer is:

halo

I'm a little stuck... Click here to teach me more about this clue!

'nimbus' is the definition.
(synonyms)

'with a long centre' is the wordplay.
'HALO' is hidden within the letters.
But, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for halo that I've seen before include "Luminous sign of saintliness" , "Aura representing saintliness" , "This indicates holiness" , "glory" , "Saint's headlight?" .)

I've seen this clue in the Sydney Morning Herald.

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