Cleaner left sand outside (6)
I believe the answer is:
bleach
'cleaner' is the definition.
(I've seen this before)
'left sand outside' is the wordplay.
'left' becomes 'l' (common abbreviation).
'sand' becomes 'beach' (sand can refer to a sandy beach).
'outside' means one lot of letters goes inside another.
'l' going within 'beach' is 'BLEACH'.
(Other definitions for bleach that I've seen before include "Whitening agent" , "Whiten by sunlight or chemicals" , "Stuff that whitens, hair or cloth" , "Clean" , "Whitening chemical" .)