The most common and most likely answer to this crossword clue is the 10 letter word COMMON-LAW
I've seen this crossword clue in Financial Times
C
O
M
M
O
N
-
L
A
W
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The straight part
We know that "partner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case partner = common-law.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 COMMON-LAW mean?
COMMON-LAW adjective- Of or pertaining to common law.
- Relating to common-law marriage.