Actually happen to see astronomical phenomenon over French street (4,4)
I believe the answer is:
come true
'actually happen' is the definition.
'come true' can be an answer for 'happen' (if something comes true it really happens). I am not sure about the 'actually' bit.
'astronomical phenomenon over french street' is the wordplay.
'astronomical phenomenon over' becomes 'comet' (I can't explain this - if you can you should believe this answer much more).
'french street' becomes 'rue' (French for street or road).
'comet'+'rue'='COME TRUE'
'to see' acts as a link.
Can you help me to learn more?
(Other definitions for come true that I've seen before include "Turn out to be accurate" , "Materialise" , "Happen , be fulfilled" , "Happen in the way that was hoped or forecast" , "Happen in reality, as dreams may (4, 4)" .)