He paid rat involved in authoritarian, discriminatory system (9)
I believe the answer is:
apartheid
'discriminatory system' is the definition.
(historical system in South Africa)
'he paid rat involved in' is the wordplay.
'involved in' indicates an anagram (involved can mean complex or confusing).
'he'+'paid'+'rat'='hepaidrat'
'hepaidrat' anagrammed gives 'APARTHEID'.
'authoritarian' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for apartheid that I've seen before include "divisive policy" , "former South African segregation system" , "South Africa's former social policy - hit parade (anag)" , "Tyrannical system in S Africa now abandoned" , "Defunct system of segregationist government in South Africa" .)