Aircraft witness simply reported (8)
I believe the answer is:
seaplane
'aircraft' is the definition.
(seaplane is a kind of aircraft)
'witness simply reported' is the wordplay.
'witness' becomes 'sea' (I am not sure about this - if you are sure you should believe this answer much more).
'simply' becomes 'plain' (I've seen this before).
'reported' indicates a 'sounds like' (homophone) clue.
'plain' is a homophone of 'plane'.
'sea'+'plane'='SEAPLANE'
Can you help me to learn more?
(Other definitions for seaplane that I've seen before include "Marine aircraft" , "Floating aircraft" .)