Sift to succeed at this very moment (6)
I believe the answer is:
winnow
'sift' is the definition.
(I know that winnow is a more specific form of the action sift)
'succeed at this very moment' is the wordplay.
'succeed' becomes 'win' (succeed can mean to win or excel).
'at this very moment' becomes 'now' (I've seen this in another clue).
'win'+'now'='WINNOW'
'to' is the link.
(Other definitions for winnow that I've seen before include "Sift out" , "Separate the wheat from the chaff" , "Dehusk grain using air currents" , "Fan" , "Separate chaff from grain" .)