Tries during proper dry run (9)
I believe the answer is:
rehearsal
'dry run' is the definition.
(synonyms)
'tries during proper' is the wordplay.
'tries' becomes 'hears' (as in trying or hearing in a trial).
'during' means one lot of letters goes inside another.
'proper' becomes 'real' (similar in meaning).
'hears' put into 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Drill" , "show trial" , "Play run-through" , "Dummy run" , "Practice - run-through" .)