This tends to be a style choice by an author, who uses a kanji for aesthetic/artistic purposes for a word which is usually expressed with 外来語 (borrowed words from foreign languages). The author can choose to do this for a few reasons. Often if they are concerned that the 外来語 they're using will not be understood by everyone, they use kanji to express the meaning of the word with the 外来語 as furigana. In this case ホストクラブ is a common word that would be recognised by everyone, so perhaps they did it to shorten the written title?

Note that クラブ is not an actual pronunciation of 部, and would never usually be read as such. Without the furigana to specify pronunciation this would almost certainly be read as ぶ here.

This tends to be a style choice by an author, who uses a kanji for aesthetic/readability purposes for a word which is usually expressed with 外来語 (borrowed words from foreign languages). The author can choose to do this for a few reasons. Often if they are concerned that the 外来語 they're using will not be understood by everyone, they use kanji to express the meaning of the word with the 外来語 as furigana. In this case ホストクラブ is a common word that would be recognised by everyone, so perhaps they did it to shorten the written title?

Note that クラブ is not an actual pronunciation of 部, and would never usually be read as such. Without the furigana to specify pronunciation this would almost certainly be read as ぶ here.