Deadly piste negotiated: fast time? (9)
I believe the answer is:
pestilent
'deadly' is the definition.
('pestilent' can be a synonym of 'deadly')
'piste negotiated fast time?' is the wordplay.
'negotiated' indicates an anagram.
'fast time?' becomes 'lent' (I've seen this before).
'piste' is an anagram of 'pesti'.
'pesti'+'lent'='PESTILENT'
(Other definitions for pestilent that I've seen before include "Plaguey" , "Pernicious" , "Such a serious nuisance" , "Likely to spread epidemic disease" , "Extremely harmful" .)