Runners on less firm terrain here — fine, in spite of stumbling (3,5)
I believe the answer is:
off-piste
'runners on less firm terrain here' is the definition.
('runner' could refer to a 'ski')
'fine in spite of stumbling' is the wordplay.
'fine' becomes 'f' (abbreviation).
'in' indicates putting letters inside.
'stumbling' indicates anagramming the letters.
'spite'+'of'='spiteof'
'spiteof' with letters rearranged gives 'ofpiste'.
'f' inserted within 'ofpiste' is 'OFF-PISTE'.
(Other definitions for off-piste that I've seen before include "On unauthorised ski slope" , "Away from prepared ski runs" , "Away from the regular runs in skiing" .)