Sport spent a penny investing in power (8)
I believe the answer is:
speedway
'sport' is the definition.
(I've seen this before)
'spent a penny investing in power' is the wordplay.
'spent a penny' becomes 'peed' ('pee' can be a synonym of 'spend a penny').
'investing in' means one lot of letters goes inside another.
'power' becomes 'sway' (sway is a kind of power).
'peed' inserted inside 'sway' is 'SPEEDWAY'.
(Other definitions for speedway that I've seen before include "Dirt track motorcycle sport" , "Form of motorcycle racing" , "Stadium for motorcycle racing" , "Dirt track motorcycle racing" , "Race-track" .)