Toilsome (7)
(Other definitions for laborious that I've seen before include "Involving hard work" , "Gruelling" , "Using much time and effort" , "Strenuous" , "pedestrian" .)
'toilsome' is the definition.
(I know this)
This is the entire clue.
(Other definitions for arduous that I've seen before include "Tough" , "Burdensome" , "Requiring hard work" , "Steep - hard" , "Requiring great effort" .)