Airman's plan to protect one on trial (4,5)
I believe the answer is:
test pilot
'airman's' is the definition.
(test pilot is a kind of airman)
'on trial' is the wordplay.
'on' becomes 'test' (I can't justify this - if you can you should believe this answer much more).
'trial' becomes 'pilot' ('pilot' can be a synonym of 'trial').
'test'+'pilot'='TEST PILOT'
'plan to protect one' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for test pilot that I've seen before include "Airman" , "Highly dangerous job" , "Alan Shepard once?" , "one occupying 21 [COCKPIT]" , "high flier" .)