The most common and most likely answer to this crossword clue is the 7 letter word STIFFEN
I've seen this crossword clue in The Times
S
T
I
F
F
E
N
More clues leading to the result STIFFEN
- Make firm points to clinch argument
- Get thicker senator holding argument
- Make firm series of points to clinch argument
- Tense, if sent out to gather flowers originally
- Difference between three points is set
- During lovers’ tiff, entitled to tense up suddenly
- Set tailors up on wet land
- Brace of snipe’s capital, pet — finish shortly
- Reinforce part of post if fencing
- Tense up seeing nurse cradling pet
Buddy explains!

The straight part
We know that "Make firm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case make firm = stiffen.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 STIFFEN mean?
STIFFEN verb- To make stiff.
- To become stiff.