Military unit capturing last of enemy with zero authority (3-2)
I believe the answer is:
say-so
'authority' is the definition.
(I've seen this before)
'military unit capturing last of enemy with zero' is the wordplay.
'military unit' becomes 'SAS' (Special Air Service in UK army).
'capturing' means one lot of letters goes inside another.
'last of' says to take the final letters.
'with' is a charade indicator (letters next to each other).
'zero' becomes 'O' (O resembles 0).
The last letter of 'enemy' is 'y'.
'sas' going around 'y' is 'says'.
'says'+'o'='SAY-SO'
(Other definitions for say-so that I've seen before include "Authority to make a decision" , "Mere assertion (colloq.)" , "Authority to make or allow something to happen" , "Authorisation" , "Final approval" .)