Seen in midnight sun, a mighty big wave (7)
I believe the answer is:
tsunami
'big wave' is the definition.
(a tsunami is a large wave)
'seen in midnight sun a mighty' is the wordplay.
'seen in' indicates the answer is hidden within the clue.
'TSUNAMI' is hidden in the letters of 'midnight sun a mighty'.
(Other definitions for tsunami that I've seen before include "stormy sea" , "Freak wave" , "Long sea wave" , "Seismic sea wave" , "Huge destructive sea wave" .)