Loudly panned top celebs framing poor bum, a vagrant sleeper? (12)

I believe the answer is:

noctambulist

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'sleeper?' is the definition.
(I know that noctambulist is a type of sleeper)

'loudly panned top celebs framing poor bum a vagrant' is the wordplay.
I cannot quite understand how this works, but
'top' could be 'ist' (resembles 1st) and 'ist' is present in the answer.
an anagram of 'bum' is 'mbu' which is within the answer.
'a' is within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for noctambulist that I've seen is " Useful or valuable thing".)

I've seen this clue in The Times.

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