Crossword Genius

Both arduous and difficult inside (4)

Ross

I believe the answer is:

hard

I'm a little stuck... Click here to teach me more about this clue!

'difficult inside' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

'both arduous' is the wordplay.
'HARD' is hidden in amongst the letters.
However, I'm uncertain how this is indicated.

'and' is the link.

Can you help me to learn more?

(Other definitions for hard that I've seen before include "Firm; demanding; stern" , "Complicated" , "Difficult - firm" , "Firm, rigid, road leading to foreshore" , "Difficult; severe" .)

I've seen this clue in The Mirror.
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