German prince regularly rattled the voters (10)
I believe the answer is:
electorate
'the voters' is the definition.
(I've seen this before)
'german prince regularly rattled' is the wordplay.
'german prince' becomes 'elector' (I've seen this before).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'rattled' are 'ate'.
'elector'+'ate'='ELECTORATE'
(Other definitions for electorate that I've seen before include "Those qualified to vote" , "Those entitled to vote" , "Voters as a whole" , "Voters in general" , "Constituency" .)