Order big one to be inserted (4)
I believe the answer is:
fiat
'order' is the definition.
('fiat' can be a synonym of 'order')
'big one to be inserted' is the wordplay.
'big' becomes 'fat'.
'one' becomes 'i' (Roman numeral).
'to be inserted' is an insertion indicator.
'fat' placed around 'i' is 'FIAT'.
(Other definitions for fiat that I've seen before include "mandate" , "Command" , "Decree; Italian car make" , "Arbitrary order" , "Official sanction" .)