Case of regressive fascist is for shrink (6)
I believe the answer is:
reduce
'is for shrink' is the definition.
(I've seen this before)
'case of regressive fascist' is the wordplay.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'fascist' becomes 'duce' (I can't justify this - if you can you should believe this answer much more).
'regressive' with its middle removed is 're'.
're'+'duce'='REDUCE'
Can you help me to learn more?
(Other definitions for reduce that I've seen before include "diet" , "Contract" , "Concentrate" , "Slim down or discount" , "Make or become smaller" .)