What does the cryptic crossword clue "Cup tie, if lost, will be momentous (7)" mean?

The definition is "momentous". The wordplay uses a deletion. SEMINAL (deletion) = SEMINAL; definition: "momentous" The answer is SEMINAL.

Cup tie, if lost, will be momentous (7)

Guardian Cryptic #29109 | 12 Across | 2023-06-29 flower