What does the cryptic crossword clue "Revolutionary Soviet leader beheaded in square (4)" mean?

The definition is "square". The wordplay uses acrostic. ENIN (reversal="ENIN") = NINE; definition: "square" The answer is NINE.

Revolutionary Soviet leader beheaded in square (4)

Times Cryptic #28957 | 15 Across | 2024-07-01 flower