When the NumLock key was replaced with the Clear key on Apple Keyboards, the old NumLock function was still available in some applications as Shift-Clear.
This was on an application-by-application (and sometimes document-by-document) basis. The OS does not itself support a global NumLock function. Indeed, Apple dropped support for the NumLock light, even when the keyboard has one and the application supports toggling the state.
I don't have Excel to test with, but you might want to try shift-clear while the problem document is frontmost.
