M. le Président, missing mother, drinks fizz to become resolute (4-4)

I believe the answer is:

cast-iron

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(Other definitions for cast-iron that I've seen before include "Extremely robust" , "Very strong" , "Unbreakable" , "Rigid, unyielding" , "Firm and unchangeable" .)

I've seen this clue in The Everyman puzzle in The Observer.

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