Rug has nothing as a defence feature (4)
I believe the answer is:
moat
'a defence feature' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'rug has nothing' is the wordplay.
'rug' becomes 'mat' (both are floor coverings).
'has' indicates putting letters inside.
'nothing' becomes 'o' (looks like zero - 0).
'mat' enclosing 'o' is 'MOAT'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for moat that I've seen before include "defensive construction" , "Castle surround" , "Water-filled ditch around stronghold" , "Water-filled trench around a castle" , "Defensive water" .)