Separate chaff from grain (6)
I believe the answer is:
winnow
'separate chaff from grain' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for winnow that I've seen before include "Blow air through (grain) to remove chaff" , "Separate the wheat from the chaff" , "Sift out" , "Fan" , "Dehusk grain using air currents" .)