I write a note and leave (3)
I believe the answer is:
ego
'i' is the definition.
('I' in Latin)
'a note and leave' is the wordplay.
'a note' becomes 'e' (musical note).
'and' means one lot of letters go next to another.
'leave' becomes 'go' (to go is to depart or leave).
'e'+'go'='EGO'
'write' is the link.
(Other definitions for ego that I've seen before include "Consciousness" , "Self-esteem, sense of self" , "Sense of identity" , "Part of the mind" , "Person's sense of self" .)