Attack, finding nothing in fight (5)
I believe the answer is:
foray
'attack' is the definition.
(I've seen this before)
'finding nothing in fight' is the wordplay.
'finding nothing' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'fight' becomes 'fray' (fray is a kind of fight).
'o' going into 'fray' is 'FORAY'.
Can you help me to learn more?
(Other definitions for foray that I've seen before include "Brief incursion" , "Assault" , "Raid into enemy territory" , "Depredation" , ".A sudden attack or raid" .)