Approach expert in field - afraid of losing idea if expert doesn't need me
Due to time constraints I would not be able to do this on my own so I do not have many alternatives, how should I proceed?


As the comments note, you have essentially answered your own question - by not approaching the expert, who has data and can (in theory) advance the question where you have admitted you can't, you have essentially already "lost" the idea. An undeveloped idea has no inherent value.

However, beyond that, you should be aware that most professional researchers have more ideas than they have time to pursue. Right now, for example, my planning board has a 4:1 ratio of Projects I'd Like To Work On : Projects I'm Working on. This has two implications:


You shouldn't be worried about losing your idea, because in all likelihood, they already have more ideas than they know what to do with and won't particularly be searching for more to add to that pile.
You should prepare for the very likely answer to this proposal to be "I don't have time/funding".