Rude thing to call many a reprobate (4)

I believe the answer is:

drip

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'rude thing to call' is the definition.
I can't judge whether this definition defines the answer.

'many a reprobate' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'a reprobate' becomes 'rip' (informal term for a fraud).
'd'+'rip'='DRIP'

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(Other definitions for drip that I've seen before include "Sissy" , "Flow in small drops" , "Slow leak" , "Liquid falling in small golbules, or a dull person" , "Fall in small drops as from wet clothes" .)

I've seen this clue in the Evening Standard.

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