A thief in the house (7)

I believe the answer is:

burglar

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'a thief' is the definition.
(I know that burglar is a type of thief)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for burglar that I've seen before include "Thief (entering building)" , "Thief who enters a building to steal" , "House-violating thief" , "A criminal on one's premises" , "Thieving intruder" .)

I've seen this clue in the Evening Standard.

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