Each is extremely behind note (5)
I believe the answer is:
Here is my best explanation:
'each' is the definition.
('every' can be a synonym of 'each')
'extremely behind note' is the subsidiary indication.
'extremely' becomes 'very' (I've seen this before).
'behind' says to put letters next to each other.
'note' becomes 'e' (musical note).
'very' after 'e' is 'every'.
'is' is the link.
(Other definitions for every that I've seen before include "Each (one of a set)", "All possible; the utmost", "Each single (item)", "Each one without exception", "Each of a number", "All, taken separately" and "'God bless us - - one !'".)