Group of stars once performing in dreadful rooms (9)

'group of stars' is the definition.
The definition suggests a singular noun which matches the answer.

'once performing in dreadful rooms' is the wordplay.
'performing' is an anagram indicator (the letters move around or 'perform').
'in' means one lot of letters goes inside another.
'dreadful' indicates anagramming the letters.
'once' anagrammed gives 'noce'.
'rooms' anagrammed gives 'moros'.
'noce' inserted inside 'moros' is 'MONOCEROS'.

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