Girl supporting major sinful act (6)
I believe the answer is:
bigamy
'sinful act' is the definition.
'bigamy' can be an answer for 'act' (bigamy is a kind of act). I am unsure of the 'sinful' bit.
'girl supporting major' is the wordplay.
'girl' becomes 'Amy'.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'major' becomes 'big' (I've seen this in other clues).
'amy' after 'big' is 'BIGAMY'.
Can you help me to learn more?
(Other definitions for bigamy that I've seen before include "overmatiness" , "Matrimonial offence" , "crime, perhaps" , "Instance of having two spouses" , "In law, having two spouses at the same time" .)