The most common and most likely answer to this crossword clue is the 6 letter word TIRING
I've seen this crossword clue in The Daily Telegraph
T
I
R
I
N
G
Buddy explains!

The straight part
We know that "Arduous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case arduous = tiring.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 TIRING mean?
TIRING verb- To become sleepy or weary.
- To make sleepy or weary.
- To become bored or impatient (with).
- To bore.
- To dress or adorn.
- To seize, pull, and tear prey, as a hawk does.
- To seize, rend, or tear something as prey; to be fixed upon, or engaged with, anything.
- The action of tiring.
- Bits of bone and tough organic material from a corpse given to hawks to abate their hunger.
- That tires or tire.