What does the cryptic crossword clue "Sylvester Stallone's hiding at first (4)" mean?

The definition is "at first". The wordplay uses a hidden word. Hidden in "sylvestER STallone's hiding at first" The answer is ERST.

Sylvester Stallone's hiding at first (4)

Guardian Cryptic #29131 | 29 Down | 2023-07-25 flower