Clue: Military unit capturing last of enemy with zero authority (3-2)
Answer: SAY-SO
Explanation: Definition: authority
 SAS(abbrev. for the Special Air Service, a special unit in the British Army) **containing**(capturing) **last letter of**(last of) "enemy" **plus**(with) 0(letter representing 0/zero).
Defn; …/the go-ahead.