The most common and most likely answer to this crossword clue is the 11 letter word ROCK-STEADY
R
O
C
K
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S
T
E
A
D
Y
More clues leading to the result ROCK-STEADY
Buddy explains!
The straight part
We know that "Firm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firm = rock-steady.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ROCK-STEADY mean?
ROCK-STEADY adjective- Consistent and dependable in one's behaviour or performance.