Battle of party losing head (6)
I believe the answer is:
action
'battle' is the definition.
(military action)
'party losing head' is the wordplay.
'party' becomes 'faction'.
'losing head' means to remove the first letter.
'faction' with its initial letter taken off is 'ACTION'.
'of' acts as a link.
(Other definitions for action that I've seen before include "Take proceedings" , "SECOND PART OF 15" , "claim in court" , "instruction from director" , "Operation - lawsuit" .)