A gas product of Icelandic origin (4)

I believe the answer is:

saga

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'icelandic origin' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'a gas product' is the wordplay.
'a gas' anagrams to 'SAGA'.
However, I'm not clear how this is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for saga that I've seen before include "Long heroic tale" , "Soap opera" , "Medieval Icelandic legend" , "long series of events" , "Flag associated with a" .)

I've seen this clue in the Evening Standard.

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