The most common and most likely answer to this crossword clue is the 7 plus 4 letter word WEDDING CAKE
I've seen this crossword clue in The Sunday Telegraph
W
E
D
D
I
N
G
C
A
K
E
Buddy explains!
The straight part
We know that "One’s cut after two are joined" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one’s cut after two are joined = wedding cake.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.