Under strain, guide high-flier up for trial? (4,5)
I believe the answer is:
test pilot
'high-flier up for trial?' is the definition.
(pilot in a test or trial)
'under strain guide' is the wordplay.
'under' is a charade indicator (letters next to each other).
'strain' becomes 'test'.
'guide' becomes 'pilot' (piloting is a kind of guiding).
'test' put next to 'pilot' is 'TEST PILOT'.
(Other definitions for test pilot that I've seen before include "Experimental flyer" , "highflying expert" , "Alan Shepard once?" , "One might check bombers" , "high flier" .)