Adult sin? I think so, too (6)
I believe the answer is:
agreed
'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" .)