It whitens stretch of shore round front of lake (6)
I believe the answer is:
bleach
'it whitens' is the definition.
'stretch of shore round front of lake' is the wordplay.
'stretch of shore' becomes 'beach'.
'round' means one lot of letters goes inside another.
'front of' suggests taking the first letters.
The initial letter of 'lake' is 'l'.
'beach' going around 'l' is 'BLEACH'.
(Other definitions for bleach that I've seen before include "Chemical whitener" , "Make colourless" , "Blanche" , "Whitening agent" , "Sterilising whitener" .)