Neglected study may easily become so (5)

I believe the answer is:

dusty

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'so' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'neglected study may easily' is the wordplay.
'study' can be anagrammed to 'DUSTY'.
But, I'm not sure how this is indicated.

'become' acts as a link.

Can you help me to learn more?

(Other definitions for dusty that I've seen before include "Old-fashioned" , "In need of cleaning" , "Dull (like a miller?)" , "In need of a spring clean" , "Powdery" .)

I've seen this clue in The Globe and Mail.

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