A deceptive note not easily understood (4)
I believe the answer is:
code
'a deceptive' is the definition.
I can't judge whether this defines the answer.
'note not easily understood' is the wordplay.
'note' becomes 'c' (musical note).
'not easily understood' becomes 'ode' (I can't explain this - if you can you should give a lot more credence to this answer).
'c'+'ode'='CODE'
Can you help me to learn more?
(Other definitions for code that I've seen before include "Method of secret communication" , "System used to disguise or represent words, like Morse" , "regulations" , "Message in cipher" , "Playfair is one" .)