The most common and most likely answer to this crossword clue is the 12 plus 4 letter word KNOCKING-OFF TIME
I've seen this crossword clue in The Times
K
N
O
C
K
I
N
G
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O
F
F
T
I
M
E
More clues leading to the result KNOCKING-OFF TIME
Buddy explains!
The straight part
We know that "when job's done" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when job's done = knocking-off time.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.