Tough coming together around sport (9)
I believe the answer is:
gruelling
'tough' is the definition.
'coming together around sport' is the wordplay.
'coming together' becomes 'gelling'.
'around' indicates putting letters inside.
'sport' becomes 'ru' (rugby union).
'gelling' going around 'ru' is 'GRUELLING'.
(Other definitions for gruelling that I've seen before include "Strenuous" , "Severe" , "Arduous, backbreaking" , "Demanding" .)