I imagine a linesman has called his shot out, or otherwise called something against him, by saying "it was very clear of the line." And he's saying "Very clear, my butt." Which is a politer form of "Very clear, my ass."
By repeating "Very clear" (in a scornful tone), you're saying that it's not clear at all. The "my butt" or "my ass" is an intensifying disparager – a word I just made up ... and my first paragraph is all conjecture.