Go round twice at speed (5)
I believe the answer is:
oomph
'go' is the definition.
(I've seen this before)
'round twice at speed' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'twice' indicates the doubling of some letters.
'at' is a charade indicator (letters next to each other).
'speed' becomes 'mph' (miles per hour).
'o' duplicated is 'oo'.
'oo'+'mph'='OOMPH'
(Other definitions for oomph that I've seen before include "vital quality" , "Shows excitement aplenty" , "attractiveness" , "Sexy, exciting quality" , "Exciting energy" .)