The most common and most likely answer to this crossword clue is the 9 letter word GENERATOR
I've seen this crossword clue in The Guardian
G
E
N
E
R
A
T
O
R
Buddy explains!

The straight part
We know that "dynamo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dynamo = generator.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 GENERATOR mean?
GENERATOR noun- One who, or that which, generates, begets, causes, or produces.
- A piece of apparatus, equipment, etc, to convert or change energy from one form to another.