Stretch of water (5)
I believe this is a double definition.
'stretch' is the first definition.
(synonyms)
'water' is the second definition.
(I have seen 'open water' mean 'reach' so perhaps 'water' could also mean 'reach')
'of' is the link.
(Other definitions for reach that I've seen before include "Succeed in getting to" , "range" , "Hit" , "Grasp" , "Attain - make contact with" .)
'stretch of water' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for strait that I've seen before include "Narrow stretch of sea" , "'Bering or Gibraltar, say (6)'" , "Sound" , "Narrow stretch of water as at Gibraltar" , "Narrow kind of traits" .)