Arduous, setting about contact sport (9)
I believe the answer is:
gruelling
'arduous' is the definition.
(I know that arduous can be written as gruelling)
'setting about contact sport' is the wordplay.
'setting' becomes 'gelling' (both can mean hardening).
'about' means one lot of letters goes inside another.
'contact sport' becomes 'RU' (rugby union).
'gelling' going around 'ru' is 'GRUELLING'.
(Other definitions for gruelling that I've seen before include "Arduous, backbreaking" , "Strenuous" , "Severe" , "Demanding" .)