The state of Ontario water? (4)
I believe the answer is:
iowa
'the state' is the definition.
(Iowa is an example)
'of ontario water?' is the wordplay.
'of' indicates a hidden word.
'IOWA' can be found hidden inside 'ontario water'.
(Other definitions for iowa that I've seen before include "part of United States" , "US state, capital Des Moines" , "Midwest US state" , "'Hawkeye State'" , "US state, three vowels" .)