Brown river coming out of mountain lake (3)
I believe the answer is:
tan
'brown' is the definition.
(tan is a kind of brown)
'river coming out of mountain lake' is the wordplay.
'river' becomes 'r'.
'coming out of' is a deletion indicator.
'mountain lake' becomes 'tarn' (I've seen this before).
'tarn' with 'r' taken away is 'TAN'.
(Other definitions for tan that I've seen before include "The colour of sunburn" , "Make hide into leather - nicely brown" , "Light brown colour" , "Brown to beat" , "Convert hide to leather" .)