Clue: Your old group put up hoarding in sea once (6)
Answer: TETHYS
Explanation: Definition: sea once

**<=SET** ("group", *put up*) *hoarding ***THY **("old" word for "your")
 

 
*Tethys, named after a Greek sea goddess, was the name of the sea that separated the two great supercontinents of Laurasia and Gondwanaland after Pangaea split in two, so was roughly the predecessor of the Mediterranean.