Cream for sweetheart on diet (5)
I believe the answer is:
elite
'cream' is the definition.
(thesaurus)
'sweetheart on diet' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'on' says to put letters next to each other.
'diet' becomes 'lite' ('lite' can be a synonym of 'diet').
'e'+'lite'='ELITE'
'for' acts as a link.
(Other definitions for elite that I've seen before include "Aristocracy" , "Exclusive set" , "Chosen ones" , "Choice; best" , "A privileged or superior social group" .)