Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
instīgātor, ōris, m. [instigo], a stimulator, instigator (post-Aug.): sibi quisque dux et instigator, Tac. H. 1, 38: superfluus, Aus. Ep. 17 fin.: accusationis, Dig. 3, 2, 20.