The most common and most likely answer to this crossword clue is the 7 letter word SIXTEEN
I've seen this crossword clue in The Times
S
I
X
T
E
E
N
More clues leading to the result SIXTEEN
- Observed transporting nine tons in square
- Square is next, triangle’s last to form
- Building extension on abandoned square
- Witnessed about nine heading for Tiananmen Square
- Wrong to accept kiss and clothing item like some adolescents
- 4×4 spotted carrying baseball team heading for tournament
- Nurse outside exit waving goodbye to French people in the grid?
- Unusually tense about turning 11 — an important age for an adolescent
- Number by Sen? Exit assembly...
- Number nine, peg between two poles
Buddy explains!

The straight part
We know that "square" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case square = sixteen.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 SIXTEEN mean?
SIXTEEN numeral- The cardinal number occurring after fifteen and before seventeen, represented in Arabic numerals as 16 and in Roman numerals as XVI.