Gruelling experience (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
ordeal
'gruelling experience' is the definition.
(I know that ordeal are arduous)
This is all the clue.
(Other definitions for ordeal that I've seen before include "Trial; nightmare" , "Suffering - trial" , "A difficult and harrowing experience" , "Horrific experience" , "Disturbing experience" .)