What does the cryptic crossword clue "What may be trapped in coastal garbage? (4)" mean?

The definition is "What may be trapped in coastal garbage?". The wordplay uses a hidden word. Seaweed hidden in coastAL GArbage. The answer is ALGA.

What may be trapped in coastal garbage? (4)

Times Cryptic #28882 | 1 Down | 2024-04-04 flower