The most common and most likely answer to this crossword clue is the 3 plus 4 letter word GAS MAIN
I've seen this crossword clue in The Guardian
G
A
S
M
A
I
N
More clues leading to the result GAS MAIN
Buddy explains!

The straight part
We know that "Supplier of fuel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case supplier of fuel = gas main.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.