Hide missing note: it’s froth (6)
I believe the answer is:
lather
This could be a double definition.
'hide missing note' is the first definition.
'lather' can be an answer for 'hide' (thesaurus). I'm not sure about the 'missing note' bit.
'froth' is the second definition.
(lather is a kind of froth)
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for lather that I've seen before include "Froth produced by soap" , "Whip up froth - tizzy" , "Froth formed by soap and water" , "starting to run in a sweat" , "Foam - tizzy (informal)" .)