Get on top of a series — it's going downhill fast! (6,3)
I believe the answer is:
cresta run
'it's going downhill fast' is the definition.
(type of toboggan track)
'get on top of a series' is the wordplay.
'get on top of' becomes 'crest' (crest can mean to get to the top of something).
'series' becomes 'run' (a run or sequence of things).
'crest'+'a'+'run'='CRESTA RUN'
(Other definitions for cresta run that I've seen before include "Frozen channel" , "tobogganing slope" , "Swiss skeleton toboggan track" .)