Hearing score for rehearsal ... (5,3)
I believe the answer is:
trial run
'for rehearsal' is the definition.
(I've seen this before)
'hearing score' is the wordplay.
'hearing' becomes 'trial' (as in a court case).
'score' becomes 'run' (run is a kind of score).
'trial'+'run'='TRIAL RUN'
(Other definitions for trial run that I've seen before include "Test drive" , "Test procedure - on the trot?" , "Experimental outing" , "'Preliminary test of how something works (5,3)'" , "Test, try-out" .)