Writing is simply talking to someone else on paper. Anybody who can think clearly, can write clearly - about any subject at all.
There're lots of ways to overcome your fear of business writing. But a good place to start is to recognise false ideas that can hold you back:
If you still have lingering fears of writing, just remember - there's no such thing as good writing, there's only good editing.
Just get it down - you can always make it better later.