Note enclosed in letter found by large stream (6)
I believe the answer is:
runnel
'stream' is the definition.
(runnel is a kind of stream)
'note enclosed in letter found by large' is the wordplay.
I cannot quite understand how this works, but
'note' could be 'n' (abbreviation) and 'n' is located in the answer.
'letter' could be 'rune' (I've seen this before) and 'rune' is found in the leftover letters.
'large' could be 'l' and 'l' is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
(Other definitions for runnel that I've seen before include "Small stream or gutter" , "Rivulet" , "Brook; gutter" , "A small stream or brook" .)