Separate with a sieve (4)
I believe the answer is:
sift
'separate with a sieve' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sift that I've seen before include "Separate by passing through fine mesh" , "Examine minutely" , "Get lumps from (flour)" , "Examine (eg evidence) discriminatingly" , "Make fist of filter" .)