The most common and most likely answer to this crossword clue is the 11 letter word LOCOMOTIVES
I've seen this crossword clue in The Daily Telegraph
L
O
C
O
M
O
T
I
V
E
S
Buddy explains!

The straight part
We know that "vehicles using diesel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vehicles using diesel = locomotives.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 LOCOMOTIVES mean?
LOCOMOTIVES noun- The power unit of a train that pulls the coaches or wagons.
- A traction engine
- A cheer characterized by a slow beginning and a progressive increase in speed
- A country which drives the world economy by having a high level of imports. (i.e. The United States).