Sentence:  Defective ... and faulty must be the composition in prose, which you and I with all our study and attention cannot understand. In poetry it is not exactly so; the greater part of it must be intelligible to all: but in the very best there is often an <WSD>undersong</WSD> of sense, which none beside the poetical mind, or one deeply versed in its mysteries, can comprehend. 
Meaning of the word marked with <WSD> tags:  Subordinate and underlying idea, meaning or atmosphere; undertone.