Lose lustre like small mountain lake? (7)
I believe the answer is:
tarnish
I believe this is a double definition.
'lose lustre' is the first definition.
(I've seen this before)
'like small mountain lake?' is the second definition.
(tarn can mean a lake by a mountain)
(Other definitions for tarnish that I've seen before include "Stain or discolour, say metal" , "Stain, metal or reputation" , "Stain or blemish, silver or reputation" , "Become dull and discoloured" , "Stain or blemish as with silverware" .)