They used to believe, faceless Mafia boss says (9)

I believe the answer is:

apostates

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'they used to believe' is the definition.
The definition suggests an adverb but the answer is not.

'faceless mafia boss says' is the wordplay.
'faceless' means to remove the first letter.
'mafia boss' becomes 'capo' (head of a crime syndicate).
'says' becomes 'states' ('state' can be a synonym of 'say').
'capo' with its first letter removed is 'apo'.
'apo'+'states'='APOSTATES'

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(Other definitions for apostates that I've seen before include "Those who renounce a belief" , "People who have renounced former beliefs" .)

I've seen this clue in The Daily Express.

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