Fly over East Street repeatedly (6)
I believe the answer is:
tsetse
'fly' is the definition.
(tsetse is a kind of fly)
'over east street repeatedly' is the wordplay.
'over' says the letters should be written in reverse.
'east' becomes 'e' ('E' can be a synonym of 'east').
'street' becomes 'st'.
'repeatedly' shows that letters should be duplicated.
'e'+'st'='est'
'est' back-to-front is 'tse'.
'tse' doubled is 'TSETSE'.
(Other definitions for tsetse that I've seen before include "bloodsucker" , "Bloodsucking African fly" , "African transmitter" , "fly across Africa" , "cause of sleepiness?" .)