Those starting modestly in speed event perform badly (5)
I believe the answer is:
misdo
'perform badly' is the definition.
(to misdo is to do or perform wrongly)
'those starting modestly in speed event' is the wordplay.
'those starting' suggests taking the first letters (the letters which start the words).
'event' becomes 'do' (do is a kind of event).
The first letters of 'modestly in speed' is 'mis'.
'mis'+'do'='MISDO'
(Other definitions for misdo that I've seen before include "Perform wrongly" , "Perform improperly" .)