The most common and most likely answer to this crossword clue is the 9 letter word GASOMETER
I've seen this crossword clue in The Times
G
A
S
O
M
E
T
E
R
More clues leading to the result GASOMETER
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The straight part
We know that "tank" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tank = gasometer.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 GASOMETER mean?
GASOMETER noun- An apparatus used to store or measure gas or the flow of gas, particularly in a laboratory setting.
- A large tank or reservoir for storing gas; a gasholder.