Clue: Backing group snaffled your drink once! (6)
Answer: TETHYS
Explanation: Definition: drink once!
An envelope ('snaffled') of THY ('your') in TES, a reversal ('backing') of SET ('group' – despite the mathematical distinction), for the Ocean between Laurasia and Gondwana in Mesozoic times (252 to 66 million years ago or thereabouts).