Filter, sieve (4)
I believe the answer is:
sift
I believe this is a double definition.
'filter' is the first definition.
(I've seen this in another clue)
'sieve' is the second definition.
(I know that sieve can be written as sift)
(Other definitions for sift that I've seen before include "Sieve, examine closely" , "Distinguish as with a riddle" , "Examine (eg evidence) discriminatingly" , "Pass though a sieve" , "Make fist of filter" .)