Charlie Sheen in audition for group (7)

I believe the answer is:

cluster

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'audition for group' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'charlie sheen' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'sheen' becomes 'luster' (synonyms).
'c'+'luster'='CLUSTER'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for cluster that I've seen before include "group together" , "Closely-spaced group" , "Come together" , "Small group" , "Tight group of like things" .)

I've seen this clue in The Independent.

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