I believe you want the conveniently-named subtract function, which exists for exactly the reason you've discovered:
subtract :: Num a => a -> a -> a
the same as flip (-).
Because - is treated specially in the Haskell grammar, (- e) is not a section, but an application of prefix negation. However, (subtract exp) is equivalent to the disallowed section.
If you wanted to write it pointfree without using a function like subtract, you could use flip (-), as the Prelude documentation mentions. But that's... kinda ugly.
