CLC                       CLC Clear carry flag                        CLC

  Operation:  0 -> C                                    N V - B D I Z C
                                                        . . . . . . . 0

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   CLC                 |   $18   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

What it does: Clears the carry flag. (Puts a 0 into it.)

Major uses: Always used before any addition (ADC). If there are to be a series of additions (multiple-byte addition), only the first ADC is preceded by CLC since the carry feature is necessary. There might be a carry, and the result will be incorrect if it is not taken into account.
The 6502 does not offer an addition instruction without the carry feature. Thus, you must always clear it before the first ADC so a carry won't be accidentally added.