How one may sin? Exactly (6)

I believe the answer is:

indeed

'exactly' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for indeed that I've seen before include "Really - what a surprise!" , "Certainty" , "even" , "In reality" , "Truthfully, really" .)

I've seen this clue in The Times.

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