Romeo in appropriate flying machine (5)
I believe the answer is:
drone
'flying machine' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'romeo in appropriate' is the wordplay.
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'appropriate' becomes 'done' (I can't explain this - if you can you should give a lot more credence to this answer).
'r' put into 'done' is 'DRONE'.
Can you help me to learn more?
(Other definitions for drone that I've seen before include "Remote controlled aircraft" , "RPV" , "Male honey bee, an idler" , "Bagpipe pipe" , "Queen bee1s mate" .)