Inside, both arduous and difficult (4)
I believe the answer is:
hard
'difficult' is the definition.
(I know that difficult can be written as hard)
'inside both arduous and' is the wordplay.
'inside' says the answer is hidden in the clue.
'HARD' is hidden in the letters of 'both arduous and'.
(Other definitions for hard that I've seen before include "Difficult - firm" , "Difficult; severe" , "Emergency lane" , "Complicated" , "Firm; demanding; stern" .)