Arsonist originally restricted by appropriate court order (4)
I believe the answer is:
fiat
'court order' is the definition.
'fiat' can be an answer for 'order' ('fiat' can be a synonym of 'order'). I am not certain of the 'court' bit.
'arsonist originally restricted by appropriate' is the wordplay.
'originally' suggests taking the first letters.
'restricted by' indicates putting letters inside (I've seen this in other clues).
'appropriate' becomes 'fit' ('fit' can be similar in meaning to 'appropriate').
The first letter of 'arsonist' is 'a'.
'a' going into 'fit' is 'FIAT'.
Can you help me to learn more?
(Other definitions for fiat that I've seen before include "mandate" , "Decree or official permission" , "Official sanction" , "Order or decree - let it be done" , "car from Italy" .)