millis() and lcd.clear() coding

One way: Use lcd.setCursor() to go the part that you want to change. You can write spaces or whatever new data you want to show. Leave the rest of the display alone.

#include <LiquidCrystal.h>

LiquidCrystal lcd(7, 6, 5, 4, 3, 2);
void setup()
{
    Serial.begin(9600);
    lcd.begin(20, 2);
    
    // This is fixed at the first column of the top row
    lcd.print("Time = 0");
}

unsigned long secs;     // Automatically initialized to zero
unsigned long old_time; // Automatically initialized to zero

void loop()
{
    unsigned long now = millis();
    if ((now - old_time) > 1000) {
        ++secs;
        lcd.setCursor(7, 0); // Column 8 of first row
        lcd.print(secs);
        // Also show on serial monitor just for kicks
        Serial.print("Time = ");
        Serial.println(secs);
        old_time = now;
    }
}

Regards,

Dave