Plonker's wild riposte oddly ignored (5)
I believe the answer is:
idiot
'plonker's' is the definition.
(both can mean a stupid person)
'wild riposte oddly ignored' is the wordplay.
'oddly ignored' indicates alternate letters (letters in odd-numbered positions are removed).
'wild'+'riposte'='wildriposte'
The alternating letters of 'wildriposte' are 'IDIOT'.
(Other definitions for idiot that I've seen before include "'The -----'. novel by Dostoyevsky (1868)" , "Twit" , "The - (Dostoievsky)" , "Simpleton" , "Silly ass" .)