The most common and most likely answer to this crossword clue is the 9 plus 4 letter word FRUITLESS TASK
I've seen this crossword clue in The Daily Telegraph
F
R
U
I
T
L
E
S
S
T
A
S
K
Buddy explains!
The straight part
We know that "A job done for nothing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a job done for nothing = fruitless task.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.