Breaking up thoroughfare to make water course (5)
I believe the answer is:
sweet
'course' is the definition.
(I know that sweet is a type of course)
'breaking up thoroughfare to make water' is the wordplay.
'breaking up' indicates putting letters inside (inserted letters slice into or 'break up' the word).
'thoroughfare' becomes 'st' (I've seen this in other clues).
'to make water' becomes 'wee' ('wee' can be a synonym of 'make water').
'st' going around 'wee' is 'SWEET'.
(Other definitions for sweet that I've seen before include "bon-bon" , "(see 15 across) A Chinese sauce" , "Gentle" , "Toffee, e.g." , "delightful" .)