The most common and most likely answer to this crossword clue is the 8 letter word DRUDGING
I've seen this crossword clue in Independent
D
R
U
D
G
I
N
G
Buddy explains!
The straight part
We know that "Working hard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case working hard = drudging.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 DRUDGING mean?
DRUDGING verb- To labour in (or as in) a low servile job.