Leave quietly, seeing thief unconscious (5,3)

I believe the answer is:

sneak out

'leave quietly' is the definition.
('sneak out' can be a synonym of 'leave quietly')

'thief unconscious' is the wordplay.
'thief' becomes 'sneak' (sneak thief is a kind of thief).
'unconscious' becomes 'out' ('out' can be similar in meaning to 'unconscious').
'sneak'+'out'='SNEAK OUT'

'seeing' acts as a link.

(Another definition for sneak out that I've seen is " Leave quietly".)

I've seen this clue in The Telegraph.

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