Fit source of water (4)
I believe the answer is:
well
I believe this clue is a double definition.
'fit' is the first definition.
(associated in meaning)
'source of water' is the second definition.
(water may come from a well)
(Other definitions for well that I've seen before include "rightly" , "Very much; healthy" , "boring" , "Waterhole; healthy" , "We shall" .)