Merging client names into orders using RxJS
Looks good. Yeah, sometimes APIs are hard to work with.

Instead of the loop clientsData.forEach, you might want to reduce the client data down to an object, where the keys are the client id (ie. index on the id). Then, map through your orders, and use this object to add in the client name. This will be O(N) vs. O(N^2) 

I'd also probably use a different object for orders vs. the orders with the name... just to avoid mutation, which I didn't expect... This is obviously more a style question, though. I favor immutable code, especially when it's more functional like this.