Opposing erstwhile bolshiness, ultimately they fall in line (10)

I believe the answer is:

conformers

'they fall in line' is the definition.

'opposing erstwhile bolshiness ultimately' is the wordplay.
'opposing' becomes 'con'.
'erstwhile' becomes 'former' ('former' can be a synonym of 'erstwhile').
'ultimately' indicates one should take the final letters.
The final letter of 'bolshiness' is 's'.
'con'+'former'+'s'='CONFORMERS'

I've seen this clue in The Telegraph.

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