Adverb taking the 'to' particle, Taru adjective
vigilantly (watching for an opportunity); eagerly; with an eagle eye
Yojijukugo (four character compound)
彼は競争相手を落とし入れようと虎視眈々としている。He watches for an opportunity to intrigue against his rival.
Also written 虎視眈眈【こしたんたん】