What does the cryptic crossword clue "I’m surprised youth hostel’s in stream (2,4)" mean?

The wordplay uses a container (one word inside another). YH (synonym="YH") in BECK (synonym="BECK") = BYHECK The answer is BYHECK.

I’m surprised youth hostel’s in stream (2,4)

Guardian Cryptic #29635 | 1 Down | 2025-03-06 flower