Constellation or atom (5)

I believe the answer is:

orion

'constellation' is the definition.
(I know that Orion is a constellation)

'or atom' is the wordplay.
'atom' becomes 'ion' (ion is a kind of atom).
'or'+'ion'='ORION'

(Other definitions for orion that I've seen before include "Constellation with seven bright stars" , "Great hunter of Greek legend and a constellation" , "Hunter slain by Artemis" , "Mythical great hunter" , "Constellation in the form of a belt" .)

I've seen this clue in the Evening Standard.

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