Formation of the ruling class (13)
I believe the answer is:
establishment
This clue is a double definition.
'formation' is the first definition.
(I know that formation can be written as establishment)
'ruling class' is the second definition.
(thesaurus)
'of the' acts as a link.
(Other definitions for establishment that I've seen before include "Permanent staff - powerful class" , "Institution; (the) ruling classes" , "powers that be" , "Formation" , "Governing body" .)