The most common and most likely answer to this crossword clue is the 4 plus 4 letter word KNEE JERK
I've seen this crossword clue in The Times
K
N
E
E
J
E
R
K
More clues leading to the result KNEE JERK
Buddy explains!
The straight part
We know that "Unthinking reaction to tap in surgery?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unthinking reaction to tap in surgery? = 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