What does the cryptic crossword clue "I'm surprised garden barrier's lost a little height (3)" mean?

The definition is "I’m surprised". The wordplay uses a deletion. AHA (deletion) = AHA; definition: "I'm surprised" The answer is AHA.

I'm surprised garden barrier's lost a little height (3)

Times Cryptic #28839 | 25 Across | 2024-02-14 flower