Plain water (4)
I believe the answer is:
mere
This clue is a double definition.
'plain' is the first definition.
(similar in meaning)
'water' is the second definition.
(mere is a kind of water)
(Other definitions for mere that I've seen before include "Nothing more than a small pond" , "of no significance" , "bathing may be permitted in this?" , "Pure; lake" , "Simply, nothing more than" .)