Note intended to keep detective (7)
I believe the answer is:
mediant
'note' is the definition.
(type of musical note)
'intended to keep detective' is the wordplay.
'intended' becomes 'meant' (meaning is a kind of intending).
'to keep' indicates putting letters inside.
'detective' becomes 'di' (Detective Inspector abbreviation).
'meant' going around 'di' is 'MEDIANT'.
(Other definitions for mediant that I've seen before include "Third tone of a scale" , "Third note of diatonic scale" .)