The most common and most likely answer to this crossword clue is the 6 letter word CARHOP
I've seen this crossword clue in Independent
C
A
R
H
O
P
More clues leading to the result CARHOP
Buddy explains!

The straight part
We know that "Complain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case complain = carhop.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 CARHOP mean?
CARHOP noun- A waiter or waitress who serves customers, especially in their vehicles, at a drive-in restaurant, sometimes on rollerskates.
- To work as a carhop.