The most common and most likely answer to this crossword clue is the 3 plus 2 plus 1 plus 6 letter word ALL OF A DITHER
A
L
L
O
F
A
D
I
T
H
E
R
Buddy explains!
The straight part
We know that "trembling severely" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trembling severely = all of a dither.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.