Sentence:  Not only is Mr. Frelinghuysen a member of the House Foreign Affairs Committee , but he is the grandson of the man who <WSD>was</WSD> instrumental in opening relations between the United States and Korea , Frederick T. Frelinghuysen , secretary of state in the administration of Chester A. Arthur . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)