Extremely sick, I go downhill fast (3)
I believe the answer is:
ski
'go downhill fast' is the definition.
(I've seen this before)
'extremely sick i' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'sick' with its middle taken out is 'sk'.
'sk'+'i'='SKI'
(Other definitions for ski that I've seen before include "Journey" , "What one might do in Alps" , "the end for many Russians" , "Piece of sports equipment" , "Move over snow on runners" .)