Tom comes up with a form of defence (4)
I believe the answer is:
moat
'form of defence' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'tom comes up with a' is the wordplay.
'comes up' is a reversal indicator (in a down clue, letters go up).
'with' means one lot of letters goes inside another.
'tom' backwards is 'mot'.
'mot' enclosing 'a' is 'MOAT'.
Can you help me to learn more?
(Other definitions for moat that I've seen before include "Trench around fortifications" , "Ditch round castle" , "Water-filled trench around a castle" , "Defensive water-filled ditch" , "Castle's ditch" .)