Source of down clue I derived to some extent (5)
I believe the answer is:
eider
'source of down' is the definition.
(I've seen this in another clue)
'clue i derived to some extent' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'EIDER' is hidden in the letters of 'clue i derived'.
(Other definitions for eider that I've seen before include "ie Red (anag.)" , "I've noted down" , "Duck from which down is obtained" , "Quilt- furnishing duck" , "Reed I make into a duck" .)