se sei sotto windows:
codice:
#include <windows.h>
#include <stdio.h>

void clrscr()
{
    COORD coordScreen = { 0, 0 }; 
    DWORD cCharsWritten; 
    CONSOLE_SCREEN_BUFFER_INFO csbi; 
    DWORD dwConSize; 
    HANDLE hConsole = GetStdHandle(STD_OUTPUT_HANDLE); 
    GetConsoleScreenBufferInfo(hConsole, &csbi); 
    dwConSize = csbi.dwSize.X * csbi.dwSize.Y; 
    FillConsoleOutputCharacter(hConsole, TEXT(' '), dwConSize, coordScreen, &cCharsWritten); 
    GetConsoleScreenBufferInfo(hConsole, &csbi); 
    FillConsoleOutputAttribute(hConsole, csbi.wAttributes, dwConSize, coordScreen, &cCharsWritten); 
    SetConsoleCursorPosition(hConsole, coordScreen); 
}

int main()
{
    //
    clrscr();
    //
}
oppure
utilizzi un comando (ad hoc)
includi:
#include <stdlib.h> // dichiara system()

e poi in main():

system("cls");

se sei su linux:

system("clear");