International W united original flyer (4,5)
I believe the answer is:
test pilot
'flyer' is the definition.
(test pilot is a kind of flyer)
'international w united original' is the wordplay.
'international w' becomes 'test' (I can't justify this - if you can you should give a lot more credence to this answer).
'united' means one lot of letters go next to another.
'original' becomes 'pilot' (synonyms).
'test'+'pilot'='TEST PILOT'
Can you help me to learn more?
(Other definitions for test pilot that I've seen before include "highflying expert" , "high flier" , "Alan Shepard once?" , "Stop title (anag.)" , "one examining details of flight, spin, etc" .)