New diner is less cluttered (6)
I believe the answer is:
neater
'less cluttered' is the definition.
(neatness is the opposite of clutter)
'new diner' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'diner' becomes 'eater' (to dine is to eat a meal).
'n'+'eater'='NEATER'
'is' is the link.
(Other definitions for neater that I've seen before include "Less messy" , "Tidier" , "More spick and span" , "Better" , "More tidy and orderly" .)