The best chocolate finally with fewer calories (5)
I believe the answer is:
elite
'the best' is the definition.
(elite can mean the very best of a group)
'chocolate finally with fewer calories' is the wordplay.
'finally' indicates one should take the final letters.
'with fewer calories' becomes 'lite' (I've seen this before).
The final letter of 'chocolate' is 'e'.
'e'+'lite'='ELITE'
(Other definitions for elite that I've seen before include "A particular typesize" , "Letter size; superior group" , "type in pool?" , "A privileged or superior social group" , "Identified as the best" .)