My personal feelings on the subject are that Wendel Clark would not only beat Brady Tkachuk in a fight, but absolutely annihilate him and put him on the highlight reel. Wendel not only played in a far more violent era, but he was one of the most fearsome combatants during that era of hockey.
Clark's career was unfortunately cut short because of a long list of injuries catching up to him, but he managed to rack up an impressive 116 fights during his NHL career, including 108 fights as a member of the Toronto Maple Leafs.
Tkachuk is still young, with time on his side and a lot of games ahead of him, but with just 27 fights to his credit, he'd be like a deer in headlights if Wendel Clark was staring him down. Not only do I think such a fight wouldn't even be close, but I'd be willing to bet that if Tkachuk fought Clark even once in his prime, he wouldn't dare to do it again.