Whereon sportsmen can possibly step out of line? (5)
I believe the answer is:
piste
'line?' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for piste that I've seen before include "slippery slope" , "Ski-slope" , "Ski track" , "Skiing trail" , "Skiing course" .)