Light conversation? (3)

I believe the answer is:

gas

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'light' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for gas that I've seen before include "See 2" , "Piped fuel" , "Hydrogen or oxygen, say" , "Wind" , "In brief, American petrol" .)

I've seen this clue in the Evening Standard.

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