Preserve a large waterway (5)
I believe the answer is:
canal
'waterway' is the definition.
(I know that canal is a type of waterway)
'preserve a large' is the wordplay.
'preserve' becomes 'can' (canning is a kind of preserving).
'large' becomes 'L' (eg in clothes sizes).
'can'+'a'+'l'='CANAL'
(Other definitions for canal that I've seen before include "Manmade flower" , "Artificial waterway as at Panama (5)" , "Grand Union perhaps" , "Man-made strip of water" , "Long, narrow strip of water" .)