Drain to make white, not black (5)
I believe the answer is:
'drain' is the definition.
(I've seen this before)
'make white not black' is the wordplay.
'make white' becomes 'bleach' (to bleach something can mean to make it white).
'not' indicates named letters should be taken away.
'black' becomes 'b' (as in pencil classifications).
'bleach' with 'b' taken away is 'LEACH'.
'to' is the link.
(Other definitions for leach that I've seen before include "Filtrate" , "Drain away through percolating liquid" , "Remove (soluble material) by percolation" , "Make liquid percolate through soil" , "Make liquid percolate through a solid" .)