Good puzzle! I watching you learning to set cryptics in real time.
I think the convention for clues like 5A would be to have an indicator that Jack is only J such as “Movie Jack initially attaches sounds of sympathy” or “Movie Jack at first attaches sounds of sympathy”. Unless the one letter is a standard abbreviation such as R for right or B for bishop.
I’m not saying to change your clue, just trying to share my understanding of convention for future puzzle-making
