Faction starts to collapse acrimoniously: punch-ups on regular basis to follow! (6)
I believe the answer is:
caucus
'faction' is the definition.
(thesaurus)
'starts to collapse acrimoniously punch-ups on regular basis to follow' is the wordplay.
'starts to' indicates taking the first letters.
'on regular basis' indicates alternate letters (letters selected at regular intervals).
'to follow' is a charade indicator (letters next to each other).
The first letters of 'collapse acrimoniously' is 'ca'.
'punch'+'ups'='punchups'
The alternate letters of 'punchups' are 'ucus'.
'ca'+'ucus'='CAUCUS'
(Other definitions for caucus that I've seen before include "Political gathering to select a candidate, US-style" , "Meeting of party leaders" , "Policy meeting" , "(US) party meeting" , "Small group of like-minded politicians" .)