Authority over soldier (8)
I believe the answer is:
commando
'soldier' is the definition.
(I've seen this before)
'authority over' is the wordplay.
'authority' becomes 'command' (command is a kind of authority).
'over' becomes 'o' (cricket abbreviation**).
'command'+'o'='COMMANDO'
(Other definitions for commando that I've seen before include "Specialist soldier" , "Shock troop unit" , "One soldier" , "Soldier from elite unit" , "going this way is unbriefed?" .)