The most common and most likely answer to this crossword clue is the 6 letter word COUPLE
I've seen this crossword clue in The Guardian
C
O
U
P
L
E
Buddy explains!
The straight part
We know that "duo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case duo = couple.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 COUPLE mean?
COUPLE noun- Two partners in a romantic or sexual relationship.
- Two of the same kind connected or considered together.
- A small number.
- One of the pairs of plates of two metals which compose a voltaic battery, called a voltaic couple or galvanic couple.
- Two forces that are equal in magnitude but opposite in direction (and acting along parallel lines), thus creating the turning effect of a torque or moment.
- A couple-close.
- That which joins or links two things together; a bond or tie; a coupler.
- To join (two things) together, or (one thing) to (another).
- To join in wedlock; to marry.
- To join in sexual intercourse; to copulate.
- Two or (a) small number of.