Attempt perhaps to streamline assembly (4)
I believe the answer is:
diet
I believe this clue is a double definition.
'attempt perhaps to streamline' is the first definition.
(a diet is an attempt to slim down)
'assembly' is the second definition.
(term for a parliamentary assembly in some nations)
(Other definitions for diet that I've seen before include "Legislative assembly; food regimen" , "Restrict food-intake" , "Supposed slimming plan" , "Slimming regime" , "Slimmer's regimen" .)