Sublime Text - Go to definition

You'll want to find out the command that is represented by this menu entry (using sublime.log_commands() in the console) and then create a mousemap file in the packages/User, according to this reference.

[
  {
    "button": "button1",
    "count": 1,
    "modifiers": ["option"],
    "press_command": "drag_select",
    "command": "lsp_symbol_definition"
  }
]