Horse around with Les, perhaps, shamefully (6)

I believe the answer is:

basely

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'shamefully' is the definition.
Both the answer and definition are adverbs. Maybe there's a link between them I don't understand?

'horse around with les perhaps' is the wordplay.
'horse around' becomes 'bay' (bay is a kind of horse. I am not sure about the 'around' bit.).
'with' indicates putting letters inside.
'perhaps' indicates an anagram.
'les' anagrammed gives 'sel'.
'bay' placed around 'sel' is 'BASELY'.

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I've seen this clue in the Evening Standard.

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