Sentence:  Several times it came near breaking , and there were in fact some lovely peals of thunder from Jerry Mulligan 's big band , which <WSD>is</WSD> about as fine an aggregation as has come along in the jazz business since John Hammond found Count Basie working in a Kansas City trap . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)