Compulsive complaining about party's leader (8)
I believe the answer is:
gripping
'compulsive' is the definition.
(thesaurus)
'complaining about party's leader' is the wordplay.
'complaining' becomes 'griping' (griping is a kind of complaining).
'about' means one lot of letters goes inside another.
'leader' says to take the initial letters.
The initial letter of 'partys' is 'p'.
'griping' enclosing 'p' is 'GRIPPING'.
(Other definitions for gripping that I've seen before include "Suspenseful" , "Grasping" , "Holding tightly" , "Enthralling" , "as one's vices may be" .)