THE matter has to be addressed sooner or later and Keeping Posted intends to do it now. How are we supposed to address Anson Chan when, as Chief Secretary, she assumes the role of Acting Governor whenever Chris Patten is out of town? We know it is a question that is concentrating minds on both levels of Albert Road. When we put it to a couple of Government House officials in recent days the response was a furrowed brow. The term ''Governess'' comes readily to mind. But admittedly that smacks of someone heading a private school for girls, or even presiding over a nursery at a stately aristocratic abode. So we turned for enlightenment to Mike Hanson, the amiable press secretary to the Governor - and he held no such doubts. ''The correct term is Governor, of course,'' he stressed. ''The Governor is sexless.'' So now we know.