The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 plus 4 letter word WIFE IN NAME ONLY
I've seen this crossword clue in Financial Times
W
I
F
E
I
N
N
A
M
E
O
N
L
Y
Buddy explains!
The straight part
We know that "one’s not truly married" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one’s not truly married = wife in name only.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.