Ninny (5)
'ninny' is the definition.
(I know that ninny can be written as idiot)
This is all the clue.
(Other definitions for idiot that I've seen before include "Goose" , "Dostoevsky hero" , "Moron, fool" , "Dipstick" , "Clot" .)
(Other definitions for twerp that I've seen before include "Ninny" , "Silly person" , "dope" , "Foolish person (slang)" , "Pillock" .)