Sentence:  But that year was different , for just as the city , in the form of my street clothes , had intruded upon my mountain nights , so an essential <WSD>part</WSD> of the summer gave promise of continuing into the fall : Jessica and I , about to be separated not by a mere footbridge or mess-hall kitchen but by the immense obstacle of residing in cruelly distant boroughs , had agreed to correspond . 
Meaning of the word marked with <WSD> tags:  something determined in relation to something that includes it