Koj has しいし【籡】pointing to しんし【籡・伸子】. Shinmeikai has "「簇・籡」とも書く" on its 伸子/しんし entry.
Daijr/s and meikyo have neither the reading nor the kanji. We could add 簇 as an rK form to 伸子/しんし but our only source (at least in the major refs) is shinmeikai. I don't think it's needed.

I can only see this in 日本国語大辞典 where it points to しんし.

Seems to be extremely rare. I don't think it's useful as an entry.

精選版　日本国語大辞典