The most common and most likely answer to this crossword clue is the 9 letter word GANGLIONS
I've seen this crossword clue in The Guardian
G
A
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Buddy explains!

The straight part
We know that "a nervous lot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a nervous lot = ganglions.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 GANGLIONS mean?
GANGLIONS noun- An encapsulated collection of nerve-cell bodies, located outside the brain and spinal cord.
- Any of certain masses of gray matter in the brain, as the basal ganglia.
- (by extension) A centre of intellectual or industrial force, activity, etc.
- A cystic tumour on a tendon sheath or joint capsule; a ganglion cyst.