Announce aircraft manifest (5)
I believe the answer is:
plain
'manifest' is the definition.
(I know that manifest can be written as plain)
'announce aircraft' is the wordplay.
'announce' shows a homophone (sound like).
'aircraft' becomes 'plane' (plane is a kind of aircraft).
'plane' is a homophone of 'PLAIN'.
(Other definitions for plain that I've seen before include "Clear, candid" , "Undecorated; flat area" , "open country" , "Simply" , "Basic" .)