The most common and most likely answer to this crossword clue is the 8 letter word GROANING
I've seen this crossword clue in The Guardian
G
R
O
A
N
I
N
G
Buddy explains!
The straight part
We know that "Complaining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case complaining = groaning.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 GROANING mean?
GROANING verb- To make a groan.
- To strive after earnestly, as if with groans.
- A low sound associated with extended suffering, sorrow, and toil.
- That groans.
- Heavily laden, as if to creak under the strain.