Notes barrier to security (7)
I believe the answer is:
defence
'security' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'notes barrier' is the wordplay.
'notes' becomes 'de'.
'barrier' becomes 'fence' (fence is a kind of barrier**).
'de'+'fence'='DEFENCE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for defence that I've seen before include "A means of protecting from attack" , "Evidence from the accused" , "team of lawyers" , "Justification for some act" , "Act of keeping safe from danger" .)