Stranded across lake, turned white (8)
I believe the answer is:
bleached
'turned white' is the definition.
(bleaching is a kind of turning white)
'stranded across lake' is the wordplay.
'stranded' becomes 'beached' (I've seen this in another clue).
'across' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'beached' enclosing 'l' is 'BLEACHED'.
(Other definitions for bleached that I've seen before include "Paled" , "Chemically whitened" , "Whitened or removed colour from" .)