Unsurpassed moment: much weight borne by one (6,2,4)
I believe the answer is:
second to none
'unsurpassed' is the definition.
(I've seen this before)
'moment much weight borne by one' is the wordplay.
'moment' becomes 'second moment' (synonyms).
'much weight' becomes 'ton' (unit of weight).
'borne by' is a charade indicator (letters next to each other).
'second'+'ton'+'one'='SECOND-TO-NONE'
(Other definitions for second to none that I've seen before include "Supreme, best" , "perfect" , "unbeatable" , "Nonpareil" , "Incomparable" .)