The most common and most likely answer to this crossword clue is the 9 letter word COMBINING
I've seen this crossword clue in The Sunday Telegraph
C
O
M
B
I
N
I
N
G
Buddy explains!
The straight part
We know that "Marrying" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marrying = combining.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 COMBINING mean?
COMBINING verb- To bring (two or more things or activities) together; to unite.
- To have two or more things or properties that function together.
- To come together; to unite.
- In the game of casino, to play a card which will take two or more cards whose aggregate number of pips equals those of the card played.
- To bind; to hold by a moral tie.
- The act by which things are combined or brought together.