Filter (through) (4)
(Other definitions for sift that I've seen before include "Filter, riddle" , "Pass though a sieve" , "Distinguish as with a riddle" , "Separate by passing through fine mesh" , "Use a sieve" .)
'filter through' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seep that I've seen before include "Of liquid, ooze slowly out" , "Ooze, percolate" , "Leak through an opening" , "Trickle through slowly" , "Flow or leak slowly" .)