The most common and most likely answer to this crossword clue is the 6 plus 8 letter word LUMBAR PUNCTURE
I've seen this crossword clue in The Guardian
L
U
M
B
A
R
P
U
N
C
T
U
R
E
More clues leading to the result LUMBAR PUNCTURE
Buddy explains!
The straight part
We know that "Spinal Tap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spinal tap = lumbar puncture.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.