What does the cryptic crossword clue "A grain's fermented into wine-based drink (7)" mean?

The definition is "drink". The wordplay uses an anagram. anagram of A + GRAIN'S = SANGRIA; definition: "drink" The answer is SANGRIA.

A grain's fermented into wine-based drink (7)

Guardian Cryptic #24184 | 22 Across | 2007-09-17 flower