What does the cryptic crossword clue "Unspeakable filling from fajita book (5)" mean?

The definition is "Unspeakable". The wordplay uses a hidden word. Hidden in "unspeakable filling from fajiTA BOOk" The answer is TABOO.

Unspeakable filling from fajita book (5)

Independent Cryptic #12088 | 5 Down | 2025-07-07 flower