One way to get slimmed down parliament (4)
I believe the answer is:
diet
This clue is a double definition.
'one way to get slimmed down' is the first definition.
(a diet might make someone slimmer)
'parliament' is the second definition.
(I've seen this before)
(Other definitions for diet that I've seen before include "Supposed slimming plan" , "Worms once a source of this" , "Restriction of food intake" , "Slimming regime" , "Fast - food - convention" .)