Locks up small cabin: what trespasser might do? (5,2)

I believe the answer is:

shuts in

'locks up' is the definition.
(I've seen this in another clue)

'small cabin what trespasser might do?' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cabin' becomes 'hut' (both can mean a small building).
'what trespasser might do?' becomes 'sin' (trespass can mean to sin).
's'+'hut'+'sin'='SHUTS IN'

I've seen this clue in The Times.

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