The first element of an array is usually indexed by 0 (usually here means in most modern programming languages). The second element is usually indexed by 1 etc. So the eight bit in a byte is indexed by 7, but that doesn't suddenly make it the 7th bit.
clear the 7th bit.
I wonder how common it is to see bits numbered from 1 instead of 0, because "7th bit" to me is the top one in a byte, with weight 2^7 = 128.