What does the cryptic crossword clue "Herb's unmounted nervously subject to my control? (5,4,5)" mean?

The wordplay uses an anagram. anagram of HERB'S + UNMOUNTED = UNDERONESTHUMB The answer is UNDERONESTHUMB.

Herb's unmounted nervously subject to my control? (5,4,5)

Daily Mail Cryptic #17068 | 8 Down | 2023-04-04 flower