Perhaps Trump curtailed old defence system (4)
I believe the answer is:
judo
'defence system' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'perhaps trump curtailed old' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is found within the answer.
The remaining letters 'jud' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for judo that I've seen before include "Type of Japanese wrestling" , "Martial art (not sumo)" , "Olympic combat sport" , "Martial art - Olympic sport" , "Throwing sport (from the Japanese for gentleness)" .)