Mountain lake, one quiet and dull (7)
I believe the answer is:
tarnish
'dull' is the definition.
(I've seen this before)
'mountain lake one quiet' is the wordplay.
'mountain lake' becomes 'tarn' (I've seen this before).
'one' becomes 'I' (Roman numeral).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'tarn'+'i'+'sh'='TARNISH'
'and' is the link.
(Other definitions for tarnish that I've seen before include "Stain or blemish, silver or reputation" , "Stain or blemish as with silverware" , "Cause to lose lustre, silver or reputation" , "spot" , "Stain, metal or reputation" .)