Leave sometime, not fast (5)

I believe the answer is:

exeat

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

'sometime not fast' is the wordplay.
'sometime' becomes 'ex'.
'not fast' becomes 'eat'.
'ex'+'eat'='EXEAT'

(Other definitions for exeat that I've seen before include "Pass-out" , "Permission for absence from school" , "Permission to go" , "... [23A=PASS]" , "permission to leave" .)

I've seen this clue in The Times.

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