Ski run first used in exercise lesson (5)
I believe the answer is:
piste
'ski run' is the definition.
(I've seen this in another clue)
'first used in exercise lesson' is the wordplay.
I cannot really understand how this works, but
'first' could be 'ist' (ist resembles 1st) and 'ist' is located in the answer.
'exercise' could be 'pe' (Physical Education) and 'pe' is found in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for piste that I've seen before include "Skiing run" , "Skiing trail" , "strip of fencing" , "Ski-slope" , "A ski run of compacted snow" .)