Sentence:  His letter to his daughter on the pains of growing up <WSD>is</WSD> surely as trenchant , forthright , and warmly understanding a piece of advice as ever a grown-up penned to a sensitive child , and with just the right tone of unpatronizing good humor . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)