The most common and most likely answer to this crossword clue is the 1 plus 4 plus 2 plus 4 plus 6 plus 5 letter word A BEAR OF VERY LITTLE BRAIN
I've seen this crossword clue in The Times
A
B
E
A
R
O
F
V
E
R
Y
L
I
T
T
L
E
B
R
A
I
N
Buddy explains!
The straight part
We know that "Bothered by “intervertebral”, I , with a loaf that’s poor?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bothered by “intervertebral”, i , with a loaf that’s poor? = a bear of very little brain.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.