The most common and most likely answer to this crossword clue is the 7 letter word ARDUOUS
I've seen this crossword clue in Independent
A
R
D
U
O
U
S
More clues leading to the result ARDUOUS
- Tough couple moving into Arkansas, America
- Couple in a hurry with horse going uphill
- A trick nearly takes in couple? That’s tough
- Tough couple in Arkanas, America
- Separating from popular duo usually testing
- You and me succeeding a right pair is difficult
- Difficult couple in a hurry mostly
- A dour disposition is, to us, hard to bear
- A right pair, useless, difficult
- Taxing a couple in middle of rural America
Buddy explains!

The straight part
We know that "difficult" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case difficult = arduous.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 ARDUOUS mean?
ARDUOUS adjective- Needing or using up much energy; testing powers of endurance.
- Burning; ardent
- Difficult or exhausting to traverse.