One put in large order (4)
I believe the answer is:
fiat
'order' is the definition.
('fiat' can be a synonym of 'order')
'one put in large' is the wordplay.
'one' becomes 'i' (Roman numeral).
'put in' means one lot of letters goes inside another.
'large' becomes 'fat' (I've seen this in other clues).
'i' going into 'fat' is 'FIAT'.
(Other definitions for fiat that I've seen before include "car from Italy" , "mandate" , "Order or decree - let it be done" , "Decree; Italian car make" , "Decree or official permission" .)