The most common and most likely answer to this crossword clue is the 9 letter word KNEE-JERK
I've seen this crossword clue in Financial Times
K
N
E
E
-
J
E
R
K
Buddy explains!

The straight part
We know that "Unconsciously reactive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unconsciously reactive = knee-jerk.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 KNEE-JERK mean?
KNEE-JERK noun- A sudden reflexive movement of the leg below the knee, as a reaction to a tap to the tendon just below the patella (kneecap).
- Unthinking, not carefully considered, (nearly) automatic, spontaneous, easily predictable