Shocks a party leader and mates (6)
I believe the answer is:
appals
'shocks' is the definition.
('appal' can be a synonym of 'shock')
'a party leader and mates' is the wordplay.
'party leader' becomes 'p' (I've seen this in other clues).
'and' means one lot of letters go next to another.
'mates' becomes 'pals' (I've seen this in another clue).
'a'+'p'+'pals'='APPALS'
(Other definitions for appals that I've seen before include "Fills with disgust" , "'Horrifies, outrages (6)'" , "Fills with dismay or horror (6)" , "Shocks" .)