Run in Bucharest (4)
I believe the answer is:
hare
'run' is the definition.
(hare can mean to run very fast)
'in bucharest' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HARE' is hidden within 'bucharest'.
(Other definitions for hare that I've seen before include "Rabbit-like animal" , "Leveret" , "Swift mammal" , "may be electric!" , "'Swift, long-eared animal (4)'" .)