Preserve a lake waterway (5)
I believe the answer is:
canal
'waterway' is the definition.
(I've seen this before)
'preserve a lake' is the wordplay.
'preserve' becomes 'can' (canning is a kind of preserving).
'lake' becomes 'L' (geographical abbreviation).
'can'+'a'+'l'='CANAL'
(Other definitions for canal that I've seen before include "Man-made strip of water" , "Man-made waterway" , "Long, narrow strip of water" , "Man-made watercourse" , "navigate this?" .)