Misfire in defeat, accidental (4,4)
I believe the answer is:
fall flat
'misfire' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'defeat accidental' is the wordplay.
'defeat' becomes 'fall' (fall can mean a loss or defeat).
'accidental' becomes 'flat' (I can't justify this - if you can you should give a lot more credence to this answer).
'fall'+'flat'='FALL FLAT'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for fall flat that I've seen before include "Fail to impress" , "what a bad crack will do" , "Flop" , "produce no response" , "offer no excitement" .)