Hādhā is a demonstrative subject; al-ṭawāfu can be parsed as its apposition or explanatory replacement. Huwa works as a separating/emphatic pronoun, and al-mafrūḍu is the nominative predicate.
Fī al-ḥajj is the prepositional qualifier naming the ritual context. The construction emphatically identifies this tawaf as the obligatory Hajj tawaf.