One put in large order (4)
I believe the answer is:
Here is my best explanation:
'order' is the definition.
('fiat' can be a synonym of 'order')
'one put in large' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'put in' indicates putting letters inside.
'large' becomes 'fat' (I've seen this in other clues).
'i' going within 'fat' is 'fiat'.
(Other definitions for fiat that I've seen before include "command", "Command", "Arbitrary order", "Decree or official permission", "Order or decree - let it be done", "Official permission; decree", "Decree; Italian car make", "Make of car", "car from Italy" and "Official sanction".)