Water line maybe (4)
I believe the answer is:
nile
'water' is the definition.
(Nile is an example)
'line maybe' is the wordplay.
'maybe' indicates an anagram.
'line' is an anagram of 'NILE'.
(Other definitions for nile that I've seen before include "World's longest river" , "'The longest river, blue and white (4)'" , "River of eastern Africa" , "Current" , "Take a line to the river" .)