Dash round repeatedly with speed (5)
I believe the answer is:
oomph
'dash' is the definition.
(thesaurus)
'round repeatedly with speed' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'repeatedly' indicates the doubling of some letters.
'with' means one lot of letters go next to another.
'speed' becomes 'mph' (miles per hour).
'o' duplicated is 'oo'.
'oo'+'mph'='OOMPH'
(Other definitions for oomph that I've seen before include "drive" , "Ginger" , "Exciting energy" , "sex-appeal" , "Vitality - sex appeal (informal)" .)