There are a wide number of definitions for "Hack", including a number of political applications of the word.
Imperial College Union
Within Imperial College Union, a hack is one intensely involved in union politics, usually with an unhealthy knowledge of the union's procedures or constitution. Hacks tend to form a clique, which may seem impenetrable to outsiders. Any members of the student media covering political issues are also hacks, by virtue of understanding what is going on.
In British politics, "Hack" is often used to describe a political journalist - one who understands the issues at stake and presents them to a general audience.