Best man ultimately left alone (5)
I believe the answer is:
elite
'best' is the definition.
(elite can mean the very best of a group)
'man ultimately left alone' is the wordplay.
'man' becomes 'eli' (I've seen this before).
'ultimately' suggests the final letters.
The final letters of 'left alone' are 'te'.
'eli'+'te'='ELITE'
(Other definitions for elite that I've seen before include "Letter size; superior group" , "Group enjoying superior status" , "twelve pitch in the office" , "one of the letter sizes" , "Cream, the pick" .)