A synthetic beam is working (5)

I believe the answer is:

rayon

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'a synthetic' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'beam is working' is the wordplay.
'beam' becomes 'ray' (synonyms).
'is working' becomes 'on' (on can mean working or activated).
'ray'+'on'='RAYON'

Can you help me to learn more?

(Other definitions for rayon that I've seen before include "Semi-synthetic material" , "Cellulose fabric" , "Imitation silk" , "Viscose fibre" , "Material - or any (anag)" .)

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

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