Last of the staff combat pressure group (7)
I believe the answer is:
faction
'pressure group' is the definition.
(thesaurus)
'last of the staff combat' is the wordplay.
'last of' says to take the final letters.
'combat' becomes 'action' (action can mean action in war).
The last letter of 'staff' is 'f'.
'f'+'action'='FACTION'
(Other definitions for faction that I've seen before include "group of rebels" , "band" , "Wing" , "Seeing political dissenters" , "A dissenting clique" .)