Moment heavyweight unit was top of the heap (6,2,4)

I believe the answer is:

second to none

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'top of the heap' is the definition.
I know nothing about this answer so I can't judge whether this works.

'moment heavyweight unit' is the wordplay.
'moment' becomes 'second moment' (second moment is a kind of moment).
'heavyweight' becomes 'ton' (a ton weighs a lot).
'unit' becomes 'one' (synonyms).
'second'+'ton'+'one'='SECOND-TO-NONE'

'was' acts as a link.

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(Other definitions for second to none that I've seen before include "perfect" , "Incomparable" , "certain champion" , "Unsurpassed" , "Pre-eminent" .)

I've seen this clue in the Sydney Morning Herald.

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