What makes white sand around lake (6)
I believe the answer is:
bleach
'what makes white' is the definition.
(I've seen this before)
'sand around lake' is the wordplay.
'sand' becomes 'beach' (sand can refer to a sandy beach).
'around' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'beach' placed around 'l' is 'BLEACH'.
(Other definitions for bleach that I've seen before include "Remove colour from" , "Whiten with a belch" , "Make white or faded" , "Disinfect" , "Whitening agent" .)