The most common and most likely answer to this crossword clue is the 6 letter word CARPED
I've seen this crossword clue in Financial Times
C
A
R
P
E
D
More clues leading to the result CARPED
Buddy explains!
The straight part
We know that "Complained" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case complained = carped.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 CARPED mean?
CARPED verb- To complain about a fault; to harp on.
- To say; to tell.
- To find fault with; to censure.