The most common and most likely answer to this crossword clue is the 5 letter word RIGID
I've seen this crossword clue in The Times
R
I
G
I
D
More clues leading to the result RIGID
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 = rigid.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 RIGID mean?
RIGID noun- A bicycle with no suspension system.
- Stiff, rather than flexible.
- Fixed, rather than moving.
- Rigorous and unbending.
- Uncompromising.