Doubt of using function

Hello, I have some doubt about using function.
I find that when a function is use in the first frame of the main timeline
I could even call the function at frame 15 but not the first frame.
Is that mean we need to “clear” a function which is being use before
so that to make the flash run better?
(if really can clear a function… how to clear it and where should be appopriate to put it?)
Or if i really wanna use that function later(not in first frame of the timeline) shouold i make one more same function at that particular frame or just use only one function only?
Sorry if my quetion is not clear or the thing is not goes like that …
Thanks.