The most common and most likely answer to this crossword clue is the 5 plus 5 plus 3 letter word STIFF UPPER LIP
I've seen this crossword clue in The Guardian
S
T
I
F
F
U
P
P
E
R
L
I
P
More clues leading to the result STIFF UPPER LIP
- Trait shown by English pet tucking into meal with sauce
- Hard drug has edge, but you won’t react with it
- Sign of fortitude, having quarrel with superior in error
- Cool having row, with stimulant swallowed by mistake
- When mixing ripple puff, it’s appropriate to show self-restraint
- Doggedness seen in argument with superior in error
- Self-restraint needed seeing cheek after altercation during meal?
- Demonstration of courage that’s shown while shaving?
Buddy explains!
The straight part
We know that "you won’t react with it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case you won’t react with it = stiff upper lip.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.