The first man to fly an aircraft (4,5)
I believe the answer is:
test pilot
'an aircraft' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'the first man to fly' is the wordplay.
'the first man' becomes 'test' (I can't explain this - if you can you should give a lot more credence to this answer).
'to fly' becomes 'pilot' ('pilot' can be a synonym of 'fly').
'test'+'pilot'='TEST-PILOT'
Can you help me to learn more?
(Other definitions for test pilot that I've seen before include "One might check bombers" , "one occupying 21 [COCKPIT]" , "Alan Shepard once?" , "Experimental flyer" , "high flier" .)