Sentence:  Such a Man looks upon Fame, not as a Reward of his present Virtue, but as an Earnest he has given of his future Performances; and is ashamed to <WSD>underlive</WSD> the Credit he has won, and not outshine his past illustrious Actions. 
Meaning of the word marked with <WSD> tags:  To live under; live beneath (something); to fail to live up to (something).