European diet is best (5)
I believe the answer is:
elite
'is best' is the definition.
(I've seen this before)
'european diet' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'diet' becomes 'lite' (synonyms).
'e'+'lite'='ELITE'
(Other definitions for elite that I've seen before include "A privileged class of people" , "Group considered to have superior status" , "Selected as the best" , "Select part" , "12 letters = 1 inch (type)" .)