Cryptic crossword clue
Vehicle by the road’s struggling to reach source of fuel 12

The most common and most likely answer to this crossword clue is the 12 letter word CARBOHYDRATE

I've seen this crossword clue in Financial Times
C
A
R
B
O
H
Y
D
R
A
T
E

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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.

Crossword Answer definitions

CARBOHYDRATE noun
  1. (nutrition) A sugar, starch, or cellulose that is a food source of energy for an animal or plant.
  2. (by extension, metonym) Any food rich in starch or other carbohydrates.

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