Jisho

Have you tried copy-pasting your sentence into a dictionary that can properly parse sentences and looking at the vocabulary?

• うん - yes; yeah; uh huh
• でも - but
• しなきゃ - the negative ~きゃ form of する, to do
• なんない - cannot help (doing, etc.); cannot resist; cannot bear not to (same as "ならない")
• ことがある - (for something) to happen on occasion
• んだ - adds emphasis

So basically it means roughly "yeah, but, I have to do it".

Thanks for the tip. This would be quite helpful. Can you provide the name of the dictionary, or a link to it? Thanks.

Sure. Here's a link with your particular phrase:

https://jpdb.io/search?q=うん、でも、しなきゃなんないことがあるんだ。