Boot in source of water, soggy in the end (5)
I believe the answer is:
welly
'boot' is the definition.
(short for Wellington boot)
'source of water soggy in the end' is the wordplay.
'source of water' becomes 'well' (water may come from a well).
'in the end' indicates one should take the final letters.
The last letter of 'soggy' is 'y'.
'well'+'y'='WELLY'
'in' acts as a link.
(Other definitions for welly that I've seen before include "Boot (informal)" , "showing great commitment" , "Power (inf)" , "Rubber boot (abbr)" , "One going on foot" .)