Silly fellow (5)
(Other definitions for ass that I've seen before include "Jennet" , "slow-witted person" , "Moke" , "Nincompoop" , "Horse-type animal" .)
(Other definitions for twerp that I've seen before include "Despicable person" , "mug" , "Ninny" , "Twit" , "Nitwit" .)