Both clrsc() and getch() are non-standard C functions defined in the Windows header conio.h.
getch() on it's own is also found in the Linux curses.h header.
Probably Turbo C includes these on it's own (making your code platform dependent).
OnlineGDB runs on a Linux server so your code will not run here (as your code only runs on Windows platform). You could include curses.h and remove the clrscr() call to make your code would compile. However, it would not run correctly (probably due to the way user input is sent from your browser to the server).
I'd suggest using the standard library functions, such as getchar(). This would make your code platform independent. I do not know a way of clearing the screen in a platform-independent way (unless you start using macros that would run specific code for Windows or Linux). You could consider, if you really-really need that clear screen at all.