What does the cryptic crossword clue "Deeper level supporting cast in audition (9)" mean?

The wordplay uses a homophone (sounds like). THROA (synonym="THROA") + TIER (synonym="‘level’") = THROATIER The answer is THROATIER.

Deeper level supporting cast in audition (9)

Guardian Cryptic #29640 | 7 Down | 2025-03-12 flower