Crossword Genius

In which a monarch hid his approval to imprison one? (5,3)

Ross

I believe the answer is:

royal oak

'in which a monarch hid' is the definition.
(Charles II hid in an oak tree in 1651)

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

(Another definition for royal oak that I've seen is " pub?".)

I've seen this clue in The Guardian.
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