The most common and most likely answer to this crossword clue is the 7 letter word HARDEST
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The straight part
We know that "Extremely difficult" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely difficult = hardest.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 HARDEST mean?
HARDEST adjective- (of material or fluid) Having a severe property; presenting difficulty.
- (personal or social) Having a severe property; presenting difficulty.
- Unquestionable.
- (of a road intersection) Having a comparatively larger or a ninety-degree angle.
- (of a male) Sexually aroused.
- Having muscles that are tightened as a result of intense, regular exercise.
- Plosive.
- Unvoiced
- Velarized or plain, rather than palatalized
- Having a severe property; presenting a barrier to enjoyment.
- In a physical form, not digital.
- Using a manual or physical process, not by means of a software command.
- Far, extreme.
- Of silk: not having had the natural gum boiled off.