The driving idea originally conjured up for supposed royal prerogative (6,5)
I believe the answer is:
divine right
'supposed royal prerogative' is the definition.
(divine right of kings)
'the driving idea originally conjured up' is the wordplay.
'originally' indicates taking the first letters.
'conjured up' indicates an anagram (I've seen 'conjuring up' mean this).
The first letter of 'idea' is 'i'.
'the'+'driving'+'i'='thedrivingi'
'thedrivingi' anagrammed gives 'DIVINE RIGHT'.
'for' acts as a link.
(Other definitions for divine right that I've seen before include "God-given authority" , "political doctrine" .)