Tom back with a form of defence (4)
I believe the answer is:
moat
'form of defence' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'tom back with a' is the wordplay.
'back' shows that the letters should be reversed in order.
'with' means one lot of letters goes inside another.
'tom' backwards is 'mot'.
'mot' going around 'a' is 'MOAT'.
Can you help me to learn more?
(Other definitions for moat that I've seen before include "Ditch round castle" , "A deep water-filled ditch around a castle" , "Water-filled trench around a castle" , "Castle's ditch" , "Water round a castle" .)