Preserve a lake as waterway (5)
I believe the answer is:
canal
'waterway' is the definition.
(thesaurus)
'preserve a lake' is the wordplay.
'preserve' becomes 'can' (canning is a kind of preserving).
'lake' becomes 'l'.
'can'+'a'+'l'='CANAL'
'as' is the link.
(Other definitions for canal that I've seen before include "Long, narrow strip of water" , "Artificial waterway for barges" , "Inland waterway" , "Manmade flower" , "Man-made watercourse" .)