Write down music? Never a note (6)
I believe the answer is:
notate
'write down music?' is the definition.
(as in music notation)
'never a note' is the wordplay.
'never' becomes 'not' (I've seen this in other clues).
'note' becomes 'te' (musical note as in do re mi fa so la ti).
'not'+'a'+'te'='NOTATE'
(Other definitions for notate that I've seen before include "Compose" , "Use symbols to record (eg, music)" , "Set down (in symbols)" , "Express in symbols" , "Write symbolically" .)