Mum emptied houseboat entirely? (5)

I believe the answer is:

shtum

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But I can't explain it at all! Could you help me understand?

(Other definitions for shtum that I've seen before include "silence" , "tight-lipped" , "Silent, non-communicative" , "Silent, dumb (sl.)" , "Quiet" .)

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

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