Sentence:  They did not impart this Deliberation to the Senate, for fear of making it too publick, trusting the Secret with none but Marco Antonio Barbaro, who was detained Prisoner at Constantinople; They charged him to <WSD>underfeel</WSD> the Gran Vizier, and conclude a Peace upon reasonable Conditions; and in the mean time they consulted with the rest of the Magistrates about the Subsistance of the Troops for the next Campaign. 
Meaning of the word marked with <WSD> tags:  To secretly gain knowledge from.