The most common and most likely answer to this crossword clue is the 7 letter word TOUGHEN
I've seen this crossword clue in The Daily Telegraph
T
O
U
G
H
E
N
Buddy explains!

The straight part
We know that "brace" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brace = toughen.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 TOUGHEN mean?
TOUGHEN verb- To make tough.
- To become tough.