As is, crane's insecure (7)

I believe the answer is:

arsenic

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'as' is the definition.
(As is the chemical symbol for arsenic)

'is crane's insecure' is the wordplay.
'is crane' anagrams to 'ARSENIC'.
Though, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for arsenic that I've seen before include "As briefly identified" , "Toxic metallic element used as rat poison" , "killer" , "Poison used in pesticides and weedkillers" , "Element with many poisonous compounds" .)

I've seen this clue in The Guardian.

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