Dirty like a small mountain stream? (7)
I believe the answer is:
tarnish
'dirty' is the definition.
(thesaurus)
'like a small mountain stream?' is the wordplay.
I cannot really see how this works, but
'like' could be 'ish' (similar in meaning) and 'ish' is located in the answer.
'a' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tarnish that I've seen before include "Film" , "Lose shine" , "spot" , "Besmirch" , "Go dull" .)