Each is extremely behind note (5)
I believe the answer is:
every
'each' is the definition.
(synonyms)
'extremely behind note' is the wordplay.
'extremely' becomes 'very' (I've seen this in another clue).
'behind' is a charade indicator (letters next to each other).
'note' becomes 'e' (musical note).
'very' put 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)" , "'God bless us - - one !'" , "Each one without exception" .)