The most common and most likely answer to this crossword clue is the 6 plus 3 letter word FILLER CAP
I've seen this crossword clue in The Times
F
I
L
L
E
R
C
A
P
Buddy explains!
The straight part
We know that "tanktop" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tanktop = filler cap.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.