The most common and most likely answer to this crossword clue is the 5 plus 5 plus 5 letter word MEETS ONE’S MATCH
I've seen this crossword clue in Independent
M
E
E
T
S
O
N
E
â
€
™
S
M
A
T
C
H
Buddy explains!
The straight part
We know that "Finds someone to marry, /so is no longer top dog?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case finds someone to marry, /so is no longer top dog? = meets one’s match.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.