Servilius ___, Roman tribune who struck the first blow in the assassination of Julius Caesar (5)
I believe the answer is:
casca
(Other definitions for casca that I've seen before include "Envious conspirator (J. Caesar)" , "Stabbed by him, Caesar" , "Servilius -----, senator co-assassin of Julius Caesar" , "envious assassin" , "plotter against Caesar" .)