Not pictured as sheathed (7)

I believe the answer is:

undrawn

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'sheathed' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?

'not pictured' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'pictured' becomes 'drawn' (I can't explain this - if you can you should believe this answer much more).
'un'+'drawn'='UNDRAWN'

'as' is the link.

Can you help me to learn more?

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

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