0x is the way most C compilers specify that the number is in hexadecimal.
So, 0x1 is really just 1, which looks like this in binary: 00000001
So, if you wanted to set PIN_A2 to low, and you didn't care what happened to the other pins, you could just say:
PORTA = 0;
Realistically, you do, and depending on which C compiler you're using, they may provide different ways of doing it. I use CCS C, which means I can say this:
If you're bit banging, you could do this:
PORTA &= 0b11111011;
That would clear bit 2, and leave all the other bits unchanged.
I seem to recall that may have undersireable side effects if some of the other pins are inputs, but I can't remember. Best to check with the documentation of your compiler.