You should also create a Part variable, so that you can easily define its properties that way
I’d also recommend not using the second parameter for Instance.new:
Put the LocalScript inside StarterPlayerScripts, and insert this:
local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
Mouse.Button1Down:Connect(function()
local Part = Instance.new("Part")
--Assign your properties here
Part.Parent = workspace
end)