Second piece about one going downhill (6)
I believe the answer is:
skiing
'going downhill' is the definition.
'skiing' can be an answer for 'going' (skiing is a kind of going). I also know that 'downhill' relates to this answer.
'second piece about one' is the wordplay.
'second' becomes 's' (common abbreviation).
'piece' becomes 'king' (king is a kind of piece).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'king' enclosing 'i' is 'kiing'.
's'+'kiing'='SKIING'
(Other definitions for skiing that I've seen before include "Travelling over snow on runners" , "Partaking in winter sport" , "going downhill, perhaps" , "Winter sports" , "going downhill fast" .)