The most common and most likely answer to this crossword clue is the 2 plus 7 letter word IN HARNESS
I've seen this crossword clue in The Daily Telegraph
I
N
H
A
R
N
E
S
S
Buddy explains!
The straight part
We know that "working with double horsepower?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case working with double horsepower? = in harness.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.