Responding instinctively, I sent in sweet note (9)
I believe the answer is:
pavlovian
'responding instinctively' is the definition.
(as in Pavlov's dog)
'i sent in sweet note' is the wordplay.
'sent in' is an insertion indicator.
'sweet' becomes 'pavlova' (I've seen this before).
'note' becomes 'n' (abbreviation).
'i' going into 'pavlova' is 'pavlovia'.
'pavlovia'+'n'='PAVLOVIAN'
(Other definitions for pavlovian that I've seen before include "(Of a response) involuntary" , "sort of reflex" .)