Sentence:  Thenne he cryed hym mercy and sayd Faire knyght for goddes loue slee me not / and I shall <WSD>ensure</WSD> the neuer werre ageynst thy lady / but be alwey toward her / Thenne Bors lete hym be<!--Prior quote:‘Fayre knyght, for Goddis love, sle me nat, and I shall <WSD>ensure</WSD> the never to warre ayenste thy lady, but be allway towarde hir.'--> 
Meaning of the word marked with <WSD> tags:  To make a pledge to (someone); to promise, guarantee (someone of something); to assure. (14th-18th c.)