Revolutionary part of protocol bolstered alliance (4)
I believe the answer is:
bloc
'alliance' is the definition.
(bloc is a kind of alliance)
'revolutionary part of protocol bolstered' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'part of' indicates the answer is hidden within the clue.
'colb' is hidden in the letters of 'protocol bolstered'.
'colb' back-to-front is 'BLOC'.
(Other definitions for bloc that I've seen before include "Group whose members have common objectives" , "People cooperating" , "Alliance of nations" , "Allied group of countries or political parties" , "Group acting together" .)