MH Wilds question for about Gunlance?

First of what is this icon for? During fights I noticed it deplets and builds back up yet I can't figure out what it's for?

Second the 4x Y move that's meant to end with wyrmstake cannon, this doesn't always trigger for me? Do you have to land every hit with Y for it to trigger the wyrmstake cannon move?

Third I've never managed to clash a monster? Does anyone know how clashes work?