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 "The best possible", "Each of a number", "Each single (item)", "Each one without exception", "'God bless us - - one !'", "All possible; the utmost", "As much as possible", "Each (one of a set)" and "All, taken separately".)