Killer groove in public transport (6)
I believe the answer is:
brutus
'killer' is the definition.
(assassin of Julius Caesar)
'groove in public transport' is the wordplay.
'groove' becomes 'rut' (synonyms).
'in' indicates putting letters inside.
'public transport' becomes 'bus' (bus is a kind of public transport).
'rut' inserted into 'bus' is 'BRUTUS'.
(Other definitions for brutus that I've seen before include "Honourable Roman" , ""The noblest Roman of them all"" , "associate of 24?" , "Roman conspirator" , "statesman who committed suicide in Macedonia" .)