Dirty, like a mountain lake? (7)
I believe the answer is:
tarnish
I believe this clue is a double definition.
'dirty' is the first definition.
(thesaurus)
'a mountain lake?' is the second definition.
(I've seen this before)
'like' is the link.
(Other definitions for tarnish that I've seen before include "Become dull or blemished" , "Stain or blemish, perhaps a reputation" , "Stain like silver" , "Spoil" , "Stain or blemish, silver or reputation" .)