When spread, marge's so light (8)

I believe the answer is:

gossamer

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'light' is the definition.
(synonyms)

'when spread marge's so' is the wordplay.
'marge's so' can be anagrammed to 'GOSSAMER'.
However, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for gossamer that I've seen before include "Filaments of a cobweb" , "Very fine gauze" , "Fine spider-threads" , "fine material" , "Flimsy, delicate material" .)

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