The most common and most likely answer to this crossword clue is the 11 letter word COUNTERPART
I've seen this crossword clue in The Guardian
C
O
U
N
T
E
R
P
A
R
T
Buddy explains!
The straight part
We know that "opposite number /opposite number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opposite number /opposite number = counterpart.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 COUNTERPART mean?
COUNTERPART noun- Either of two parts that fit together, or complement one another.
- A duplicate of a legal document.
- One which resembles another
- One which has corresponding functions or characteristics.
- Either half of a flattened fossil when the rock has split along the plane of the fossil.
- Counterbalance.