The best European diet (5)
I believe the answer is:
elite
'the best' is the definition.
(elite can mean the very best of a group)
'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 or superior social group" , "type in pool?" , "one of the letter sizes" , "Exclusive set" , "Cream, the pick" .)