You can nest them:
Left ( getasnumber ($ratebasisnum) ; 1)
Or you can use the let function to break it down into steps:
Let ( n = getasnumber ($ratebasisnum ; left ( n ; 1 ) )
Both of these expressions produce exactly the same results. If I'm just using two functions, I'll probably use the first option and nest one inside the other, but if I need to nest more or other factors add to the complexity of my expression, I use Let.
Nested functions evaluate from the inside out.
Let functions evaluate from left to right / top to bottom.