The most common and most likely answer to this crossword clue is the 6 letter word BOWSER
I've seen this crossword clue in The Guardian
B
O
W
S
E
R
More clues leading to the result BOWSER
Buddy explains!
The straight part
We know that "tanker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tanker = bowser.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 BOWSER mean?
BOWSER noun- A fuel metering/delivery pump at a filling station.
- A road vehicle (often a trailer) for the transport of liquid fuel, particularly aviation fuel at an airfield.
- A mobile water tank deployed to distribute fresh water in emergency situations where the normal system of piped distribution has broken down or is insufficient.
- (used in the vocative) A buffoon or imbecile.