Tax guide for one involved in aeronautics research (4,5)
I believe the answer is:
test pilot
'one involved in aeronautics research' is the definition.
(someone testing aircraft)
'tax guide' is the wordplay.
'tax' becomes 'test' (taxing can mean testing or demanding).
'guide' becomes 'pilot' (piloting is a kind of guiding).
'test'+'pilot'='TEST PILOT'
'for' acts as a link.
(Other definitions for test pilot that I've seen before include "one occupying 21 [COCKPIT]" , "Stop title (anag.)" , "high flier" , "Experimental flyer" , "Airman" .)