18 across
Excellent:
room’s
confining,
at
last,
Bruno
Mars
(7)
Excellent: room’s confining, at last, Bruno Mars (7)
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checks
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hints
The definition is at the end of the clue.
How the clue works
Mars at the end is your definition, meaning disfigures or spoils. You take DEF (slang for "excellent"), then O (the last letter of "Bruno," as signalled by "at last"), then RMS (abbreviation hiding inside "room's"), and the whole lot slots together nicely.