Adult sin? I think so, too (6)

I believe the answer is:

agreed

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'i think so too' is the definition.
I can't judge whether this definition defines the answer.

'adult sin?' is the wordplay.
'adult' becomes 'a'.
'sin?' becomes 'greed' (greed is a kind of sin).
'a'+'greed'='AGREED'

Can you help me to learn more?

(Other definitions for agreed that I've seen before include "Shared an opinion" , "Acquiesed, concurred" , "In accord" , "acceptable" , "Was fitting" .)

I've seen this clue in The Telegraph.

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