The most common and most likely answer to this crossword clue is the 7 letter word LEGWORK
I've seen this crossword clue in The Daily Telegraph
L
E
G
W
O
R
K
More clues leading to the result LEGWORK
Buddy explains!

The straight part
We know that "demanding toil" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demanding toil = legwork.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 LEGWORK mean?
LEGWORK noun- Work, especially research or preparation, that involves significant walking, travel, or similar effort.
- Skillful or vigorous use of the legs, as in dance or sports.