Grand Clan Challenges (also known as Clan Challenges) are a particular example of a regular challenge.

Clan Challenges always take place during Clan Wars. They group players from clans that are actively competing in Clan Wars.
Unlike regular challenges, Clan Challenges do not have time frames and last for almost a full day.

You enter a Clan Challenge automatically along with your clanmates and players from opposing clans at the start of the Clan Challenge.