Overture from a rebel MP vacillating (8)
I believe the answer is:
preamble
'overture' is the definition.
(thesaurus)
'a rebel mp vacillating' is the wordplay.
'vacillating' indicates anagramming the letters.
'a'+'rebel'+'mp'='arebelmp'
'arebelmp' with letters rearranged gives 'PREAMBLE'.
'from' is the link.
(Other definitions for preamble that I've seen before include "Foreword, introduction" , "Introductory explanation" , "Preliminary or introductory statement" , "Opening words" , "Make introduction" .)