Hut by lake maybe makes clear dividing line (9)
I believe the answer is:
watershed
'clear dividing line' is the definition.
(as in a watershed moment)
'hut by lake maybe' is the wordplay.
'hut' becomes 'shed' (hut in a garden).
'by' is a charade indicator (letters next to each other).
'lake maybe' becomes 'water' (body of water).
'shed' after 'water' is 'WATERSHED'.
'makes' is the link.
(Other definitions for watershed that I've seen before include "Time after which adult programmes may be shown" , "dividing line" , "Time after which content of TV programmes is less regulated" , "Line of high land from which streams flow in opposite directions" , "Turning-point in course of events, like ridge of high land" .)