One overwhelmed by blow where justice doesn't prevail (8)
I believe the answer is:
mistrial
'where justice doesn't prevail' is the definition.
(invalid legal trial)
'one overwhelmed by blow' is the wordplay.
'one' becomes 'i' (Roman numeral).
'overwhelmed by' indicates putting letters inside (inserted letters get absorbed or overwhelmed).
'blow' becomes 'mistral'.
'i' put inside 'mistral' is 'MISTRIAL'.
(Other definitions for mistrial that I've seen before include "Court hearing invalidated by error" , "damaged hearing?" , "Inconclusive or invalid court proceedings" , "When it's declared, the case must be heard again" , "Court procedure made void by error" .)