Memo attached showing defensive feature (4)
I believe the answer is:
moat
'defensive feature' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'memo attached showing' is the wordplay.
'showing' says the answer is hidden in the clue (the answer is shown in the clue).
'MOAT' is hidden within 'memo attached'.
Can you help me to learn more?
(Other definitions for moat that I've seen before include "Ditch used as a defence" , "Water crossed via a drawbridge" , "Water-filled trench around a castle" , "Defensive water" , "Ditch dug to protect a castle" .)
