Lout, attention-seeker and knucklehead (3)
I believe the answer is:
oik
'lout' is the definition.
(I've seen this before)
'attention-seeker and knucklehead' is the wordplay.
'attention seeker' becomes 'oi' (exclamation used to attract attention).
'and' is a charade indicator (letters next to each other).
'knucklehead' becomes 'k' (head letter of 'knuckle').
'oi'+'k'='OIK'
(Other definitions for oik that I've seen before include "Yob" , "anti-social type" , "Brat" , "Twerp" , "Uncouth person (colloq.)" .)