The most common and most likely answer to this crossword clue is the 5 letter word UNSET
U
N
S
E
T
Buddy explains!
The straight part
We know that "not yet firm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not yet firm = unset.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 UNSET mean?
UNSET verb- To make not set.
- Not set; not fixed or appointed.
- Not mounted or placed in a setting.
- Not set, as a broken limb.
- Not planted.