Tree in river not cold at first (5)
I believe the answer is:
alder
'tree' is the definition.
(I know that alder is a type of tree)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for alder that I've seen before include "Tree with toothed leaves and cone-like fruits" , "Tree akin to the birch" , "One [TYPE OF TREE]" , "Type of birch tree" , "Tree or shrub whose wood is resistant to underwater rot" .)