Attack or invasion for less than a day (5)
I believe the answer is:
foray
'attack' is the definition.
(foray is a kind of attack)
'or invasion for less than a day' is the wordplay.
I cannot really understand how this works, but
'or' is within the answer.
'a' is within the answer.
The remaining letters 'fy' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for foray that I've seen before include "Raid into enemy territory" , "Brief attack" , "Assault" , "Raid for plunder" , ".A sudden attack or raid" .)