Lower terribly crude note (6)
I believe the answer is:
reduce
'lower' is the definition.
(I've seen this before)
'terribly crude note' is the wordplay.
'terribly' indicates an anagram.
'note' becomes 'e' (musical note).
'crude' is an anagram of 'reduc'.
'reduc'+'e'='REDUCE'
(Other definitions for reduce that I've seen before include "Make smaller or take off weight" , "Cut down on, lessen" , "Make smaller or less" , "Shrink" , "subdue" .)