Pure water (4)
I believe the answer is:
mere
I believe this clue is a double definition.
'pure' is the first definition.
(I've seen this before)
'water' is the second definition.
(I know that mere is a type of pond)
(Other definitions for mere that I've seen before include "Pond, only what is said" , "Pierre's mother identified" , "Lake; Wiltshire town" , "Lake (poetic); Maori war club" , "Just an English lake" .)