Joins in some of the nastiest activities (4)
I believe the answer is:
ties
'joins' is the definition.
(I've seen this in another clue)
'some of the nastiest activities' is the wordplay.
'some of' says the answer is hidden in the clue.
'TIES' can be found hidden inside 'nastiest activities'.
'in' acts as a link.
(Other definitions for ties that I've seen before include "Attaches, knots" , "Drawn games" , "Binds with male neckwear" , "Items of neckwear" , "allegiances" .)