The most common and most likely answer to this crossword clue is the 11 letter word AGGLUTINATE
I've seen this crossword clue in The Times
A
G
G
L
U
T
I
N
A
T
E
Buddy explains!
The straight part
We know that "Stick together" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stick together = agglutinate.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 AGGLUTINATE mean?
AGGLUTINATE verb- To unite, or cause to adhere, as with glue or other viscous substance; to unite by causing an adhesion of substances.
- To form through agglutination.
- United with glue or as with glue; cemented together.
- Consisting of root words combined but not materially altered as to form or meaning