Stain resembling small lake? (7)
I believe the answer is:
tarnish
'stain' is the definition.
(I know that tarnish is a type of stain)
'small lake?' is the wordplay.
I cannot quite understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'lake?' could be 'tarn' (tarn is a kind of lake) and 'tarn' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'resembling' is the link.
(Other definitions for tarnish that I've seen before include "Stain like silver" , "Film" , "Stain, metal or reputation" , "Besmirch" , "(Of metal) lose lustre" .)