The most common and most likely answer to this crossword clue is the 8 letter word ALLERGIC
I've seen this crossword clue in The Sunday Telegraph
A
L
L
E
R
G
I
C
More clues leading to the result ALLERGIC
- Sensitive girl working with lace
- Garlic’s wasted round the French
- Sensitive to the French crushed garlic sandwiches
- Reacting badly to a display of ill grace
- Travel in France — US soldier getting cold is hypersensitive
- Reacting rashly perhaps to unusual ill-grace
- Sort of reaction from the French in cooking garlic?
- Reacting sensitively as some work — in charge, after all
- Soldier who’s on the phone while cycling reacting strongly
- Everyone sent to work unit? I start to cower, having an aversion
Buddy explains!

The straight part
We know that "Reacting sensitively" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reacting sensitively = allergic.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 ALLERGIC mean?
ALLERGIC noun- A person with a tendency to having allergies; a person with multiple or severe allergies.
- Of or pertaining to allergy.
- Having an allergy.
- Excessively sensitive; susceptible.