Lodger sent up cause of Casablanca’s silence? (5)

I believe the answer is:

mason

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'cause of casablanca's silence?' is the definition.
'Mason' can be an answer for 'cause' (Mason is an example). I'm not sure about the 'of casablanca's silence?' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mason that I've seen before include "Stone carver" , "Jewish comedian" , "US comedian" , "Builder in stone" , "Craftsperson who works with stone or brick" .)

I've seen this clue in the Sydney Morning Herald.

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