Authority initially over soldier (8)
I believe the answer is:
commando
'soldier' is the definition.
(I know that commando is a type of soldier)
'authority initially over' is the wordplay.
'authority' becomes 'command' (command is a kind of authority).
'initially' says to take the initial letters.
The initial letter of 'over' is 'o'.
'command'+'o'='COMMANDO'
(Other definitions for commando that I've seen before include "Royal Marine Corps soldier" , "going this way is unbriefed?" , "Military raider" , "Kilt-wearer might go" , "Soldier trained to carry out dangerous raids" .)