Special catering arrangement for assembly (4)
I believe the answer is:
diet
This clue is a double definition.
'special catering arrangement' is the first definition.
(eg a diet to lose weight)
'assembly' is the second definition.
(term for a parliamentary assembly in some nations)
'for' is the link.
(Other definitions for diet that I've seen before include "(Perhaps, try to) reduce rump; parliament" , "Fast - food - convention" , "Special food regime" , "Restricted intake" , "Calorie-watching regimen" .)