The best chocolate ultimately low in calories (5)
I believe the answer is:
elite
'the best' is the definition.
(I've seen this before)
'chocolate ultimately low in calories' is the wordplay.
'ultimately' indicates one should take the final letters.
'low in calories' becomes 'lite' (I've seen this before).
The last letter of 'chocolate' is 'e'.
'e'+'lite'='ELITE'
(Other definitions for elite that I've seen before include "Superior or favoured group of people" , "Aristocracy" , "Choice group" , "Exclusive set" , "Identified as the best" .)