Nothing at zero speed shows energy (5)
I believe the answer is:
oomph
'energy' is the definition.
(oomph can mean energy or excitement)
'nothing at zero speed' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'at' means one lot of letters go next to another.
'zero' becomes 'O' (O resembles 0).
'speed' becomes 'mph' (miles per hour).
'o'+'o'+'mph'='OOMPH'
'shows' acts as a link.
(Other definitions for oomph that I've seen before include "attractiveness" , "Get-up-and-go" , "sex-appeal" , "Go" , "Shows excitement aplenty" .)