The most common and most likely answer to this crossword clue is the 8 letter word COHESIVE
I've seen this crossword clue in The Times
C
O
H
E
S
I
V
E
Buddy explains!

The straight part
We know that "Holding it together" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case holding it together = cohesive.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 COHESIVE mean?
COHESIVE noun- A substance that provides cohesion
- A device used to establish cohesion within a text
- Having cohesion.