Laborious, gruelling (7)
I believe the answer is:
arduous
I believe this is a double definition.
'laborious' is the first definition.
(synonyms)
'gruelling' is the second definition.
(synonyms)
(Other definitions for arduous that I've seen before include "Punishing" , "Requiring hard work" , "Extremely taxing" , "Burdensome" , "Requiring great effort" .)