Sharp red line broken by libertines (12)

I believe the answer is:

philanderers

'libertines' is the definition.
(philanderer is a kind of libertine)

'sharp red line broken' is the wordplay.
'broken' indicates anagramming the letters.
'sharp'+'red'+'line'='sharpredline'
'sharpredline' is an anagram of 'PHILANDERERS'.

'by' is the link.

I've seen this clue in the Sydney Morning Herald.

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