Security barrier being defaced (7)
I believe the answer is:
defence
'security' is the definition.
(defence can mean security or protection)
'barrier being defaced' is the wordplay.
I cannot really understand how this works, but
'barrier' could be 'fence' (fence is a kind of barrier) and 'fence' is located in the answer.
The remaining letters 'de' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for defence that I've seen before include "See 12" , "Resistance to attack" , "Protection; justification" , "Military's purpose" , "Protection from harm" .)