Helper in white revised political order (5-4,4)
I believe the answer is:
three-line whip
'political order' is the definition.
(order to UK MPs to vote)
'helper in white revised' is the wordplay.
'revised' is an anagram indicator.
'helper'+'in'+'white'='helperinwhite'
'helperinwhite' with letters rearranged gives 'THREE-LINE WHIP'.
(Other definitions for three-line whip that I've seen before include "Parliamentary voting instruction" , "Urgent call to vote" , "Important instruction to MPs" , "Firm instruction to voting MPs" , "Demand for MP's vote" .)