The most common and most likely answer to this crossword clue is the 6 letter word REFLEX
R
E
F
L
E
X
More clues leading to the result REFLEX
- Involuntary response having half-read cable
- Knee-jerk reaction about electric cable
- Reference lexicon on papal treasury’s image maker
- Bent sports official on side of Liverpool once
- Retro Fairy King embraces without thinking
- Instinct makes law in Rome go after whistle-blower
- Immediate response: ‘Arbiter should be above law’
- King had run away — shortly turned backwards
- Acquire flexibility to conceal gut reaction
Buddy explains!

The straight part
We know that "Knee-jerk reaction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knee-jerk reaction = reflex.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 REFLEX mean?
REFLEX noun- An automatic response to a simple stimulus which does not require mental processing.
- The descendant of an earlier language element, such as a word or phoneme, in a daughter language.
- The descendant of anything from an earlier time, such as a cultural myth.
- Reflection or an image produced by reflection. The light reflected from an illuminated surface to one in shade.
- To bend, turn back or reflect.
- To respond to a stimulus.
- Bent, turned back or reflected.
- Produced automatically by a stimulus.
- (of an angle) Having greater than 180 degrees but less than 360 degrees.
- Illuminated by light reflected from another part of the same picture.