Both arduous and difficult within (4)
I believe the answer is:
hard
'within' is the definition.
Both the answer and definition are adverbs. Maybe you can see an association between them that I can't see?
'both arduous and difficult' is the wordplay.
'HARD' is hidden within the letters.
But, I am uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for hard that I've seen before include "Tough - laborious" , "badly" , "impenetrable" , "Newspaper's sure" , "Resentment" .)