Crime, through having bagged Yuletide threesome up (6)
I believe the answer is:
bigamy
'crime' is the definition.
(bigamy is a kind of crime)
'through having bagged yuletide threesome up' is the wordplay.
'through' becomes 'by'.
'having bagged' indicates putting letters inside.
'yuletide threesome' becomes 'magi' (I can't justify this - if you can you should believe this answer much more).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'magi' reversed gives 'igam'.
'by' going around 'igam' is 'BIGAMY'.
Can you help me to learn more?
(Other definitions for bigamy that I've seen before include "Crime involving marriage(s)" , "becoming a member of two unions?" , "Matrimonial crime" , "Instance of having two spouses" , "overmatiness" .)