That is the Horizontal output transformer, commonly known in N.A. as the Flyback.
It produces the VHV for the CRT, (9kv for monochrome, 15kv for colour).
Either the insulation has cracked and it is arcing to the adjacent components or Moisture, humidity or general dust contamination can also cause the arc across.
You need to inspect it closely and clean the transformer and the lead to the crt.
The CRT can store the DC charge for some time, so be carefull if removing it from the CRT connector.