Mountain lake is hard to discolour (7)
I believe the answer is:
tarnish
'discolour' is the definition.
(I've seen this before)
'mountain lake is hard' is the wordplay.
'mountain lake' becomes 'tarn' (I've seen this before).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'tarn'+'is'+'h'='TARNISH'
'to' acts as a link.
(Other definitions for tarnish that I've seen before include "Become dull or blemished" , "Stain or discolour, say metal" , "Spoil" , "Stain or blemish as with silverware" , "spot" .)