Operate following hearing test (5,3)
I believe the answer is:
trial run
'test' is the definition.
(I have seen 'Introductory test' mean 'trial run' so perhaps 'test' could also mean 'trial run')
'operate following hearing' is the wordplay.
'operate' becomes 'run' ('run' can be a synonym of 'operate').
'following' is a charade indicator (letters next to each other).
'hearing' becomes 'trial' (I've seen this before).
'run' after 'trial' is 'TRIAL RUN'.
(Other definitions for trial run that I've seen before include "Test drive" , "Rehearsal" , "Turn rail around in practical test" , "Preliminary evaluation like test trot" , "Test procedure - on the trot?" .)