Slim down parliament (4)
I believe the answer is:
diet
I believe this clue is a double definition.
'slim down' is the first definition.
(I've seen this in another clue)
'parliament' is the second definition.
(I've seen this in another clue)
(Other definitions for diet that I've seen before include "food one eats on a regular basis" , "provisions" , "Range of food eaten" , "Weight-loss plan" , "Choice of food" .)