Wake on high, having passed over? (6,5)
I believe the answer is:
vapour trail
'having passed over?' is the definition.
I can't tell whether this definition defines the answer.
'wake on high' is the wordplay.
'wake' becomes 'trail' (I've seen this before).
'on' means one lot of letters go next to another.
'high' becomes 'vapour' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trail' after 'vapour' is 'VAPOUR TRAIL'.
(Other definitions for vapour trail that I've seen before include "Track of aircraft in the sky" , "Line of cloud following an aeroplane" , "Visible line left in the sky by an aircraft" .)