Which involves a trio of partners in crime? (6)
I believe the answer is:
bigamy
I believe this clue is a double definition.
'which involves a trio of partners' is the first definition.
(man with two wives)
'crime?' is the second definition.
(bigamy is a kind of crime)
'in' acts as a link.
(Other definitions for bigamy that I've seen before include "Crime involving marriage(s)" , "State of having two spouses at one time" , "Marital crime" , "crime, perhaps" , "Matrimonial crime" .)