Heavenly maybe, but you'll never see Iris on one! (7)

I believe the answer is:

rainbow

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

'iris on one' is the definition.
'rainbow' can be an answer for 'iris' (I've seen this before). I am unsure of the 'on one' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for rainbow that I've seen before include "Coloured arc in the sky" , "Spectrum in the sky" , "Phenomenon produced by light passing through water droplets" , "Colourful phenomenon" , "Whole range of colours" .)

I've seen this clue in the Evening Standard.

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