I step out on trail (5)
I believe the answer is:
'trail' is the definition.
(I know that piste is a type of ski run)
'i step out on' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'step' anagrammed gives 'pste'.
'i' put within 'pste' is 'PISTE'.
(Other definitions for piste that I've seen before include "Winter sports course" , "Skiing run" , "Ski-slope" , "Skiing course" , "Skiing trail" .)