Rooms for guests of Her Majesty (5)

I believe the answer is:

cells

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'rooms' is the definition.
(cell is a kind of room)

I don't understand the rest of the clue.

(Other definitions for cells that I've seen before include "Mobile phone in the US" , "terrorist groups" , "Dungeons" , "Accommodation for prisoners" , "Basic structural units of all organisms" .)

I've seen this clue in the Evening Standard.

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