Cargo ship about to be protected by military aircraft (9)
I believe the answer is:
freighter
'cargo ship' is the definition.
(freighter is a kind of cargo ship)
'about to be protected by military aircraft' is the wordplay.
'about' becomes 're' (regarding).
'to be protected by' means one lot of letters goes inside another.
'military aircraft' becomes 'fighter' (I've seen this before).
're' put into 'fighter' is 'FREIGHTER'.
(Other definitions for freighter that I've seen before include "transport vessel" , "Cargo-ship" , "Cargo-carrying ship or plane" , "A plane carrying large load" , "Cargo ship" .)