Trust it to enter note (5)
I believe the answer is:
faith
'trust' is the definition.
(I know that trust can be written as faith)
'it to enter note' is the wordplay.
'to enter' means one lot of letters goes inside another.
'note' becomes 'fah' (fourth note in sol-fa notation, anglicized version).
'it' placed inside 'fah' is 'FAITH'.
(Other definitions for faith that I've seen before include "A woman" , "Unshakeable belief" , "Staunch trust" , "Religious belief" , "Confidence" .)