The importance of communication (4)
I believe the answer is:
note
I believe this clue is a double definition.
'the importance' is the first definition.
(I've seen this in another clue)
'communication' is the second definition.
(note is a kind of communication)
'of' is the link.
(Other definitions for note that I've seen before include "Memo; piano key" , "Quaver, say" , "Fiver, say" , "Jotting" , "Memo; pay attention to" .)