Command of royal decree exempts royals initially (5)
I believe the answer is:
order
'command' is the definition.
(order is a kind of command)
'of royal decree exempts royals initially' is the wordplay.
'of' becomes 'o''.
'initially' says to take the initial letters.
The initial letters of 'royal decree exempts royals' is 'rder'.
'o'+'rder'='ORDER'
(Other definitions for order that I've seen before include "Uniformity or fraternity" , "Rank" , "Command harmony" , "Tidiness - that's a command" , "Symmetry" .)