The housemistress’s half? (4)

I believe the answer is:

semi

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'half?' is the definition.
(I've seen this before)

'the housemistress's' is the wordplay.
'SEMI' is hidden in amongst the letters.
However, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for semi that I've seen before include "House linked to another" , "Common type of house" , "Half (pfx)" , "Property attached to another" , "Not quite the biggest tie" .)

I've seen this clue in the Evening Standard.

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