A lake, pure and simple (4)
I believe the answer is:
mere
This clue is a triple definition!
'a lake' is the first definition.
(mere is a kind of lake)
'pure' is the second definition.
(I've seen this in another clue)
'and' is a link.
'simple' is the third definition.
(synonyms)
(Other definitions for mere that I've seen before include "Simply, nothing more than" , "Bare, simple" , "Pond, only what is said" , "Previously pure" , "Lake - French mother" .)