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chance of going out; opportunity (to succeed)
musical accompaniment for an actor going on stage
「出端」 has another reading with different meanings.
出端をくじかれてしまう。
I got my momentum disrupted.
出端がよければうまくいく。
If the start goes well, things work out.
せっかくの出端を邪魔されたくない。
I don't want my opportunity to be interfered with.