Flight that can't have happened (7)
I believe the answer is:
getaway
'flight that can't' is the definition.
'getaway' can be an answer for 'flight' (getaway is a kind of flight). I am not sure about the remainder of the definition.
'have happened' is the wordplay.
'have' becomes 'get' (synonyms).
'happened' becomes 'away' (I am not sure about this - if you are sure you should believe this answer much more).
'get'+'away'='GETAWAY'
Can you help me to learn more?
(Other definitions for getaway that I've seen before include "Escape" , "flight" .)