The most common and most likely answer to this crossword clue is the 9 letter word BEMOANING
I've seen this crossword clue in Financial Times
B
E
M
O
A
N
I
N
G
Buddy explains!
The straight part
We know that "Complaining about" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case complaining about = bemoaning.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 BEMOANING mean?
BEMOANING verb- To moan or complain about (something).
- To be dismayed or worried about (someone), particularly because of their situation or what has happened to them.
- The act of one who bemoans something.