Fly east over the Channel, then again northwards (6)
I believe the answer is:
tsetse
'fly' is the definition.
(I know that tsetse is a type of fly)
'east over the channel then again northwards' is the wordplay.
'east over the channel' becomes 'est' ('east' in French).
'then again' becomes 'est' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'northwards' says the letters should be written in reverse (in a down clue, letters go up or 'north').
'est'+'est'='estest'
'estest' back-to-front is 'TSETSE'.
(Other definitions for tsetse that I've seen before include "cause of sleepiness?" , "bloodsucker" , "source of sleeping sickness" , "flies" , "Disease-carrying fly" .)