A sweet given to diner ultimately is sharper (6)
I believe the answer is:
acuter
'is sharper' is the definition.
('acute' can be a synonym of 'sharp')
'a sweet given to diner ultimately' is the wordplay.
'sweet' becomes 'cute' (cute can mean sweet or adorable).
'given to' says to put letters next to each other.
'ultimately' says to take the final letters.
The final letter of 'diner' is 'r'.
'a'+'cute'+'r'='ACUTER'
(Other definitions for acuter that I've seen before include "More sharp or shrewd" , "Sharper" .)