A foolish person (4)
(Other definitions for punk that I've seen before include "Form of rock music" , "Sex Pistols' music genre" , "Rock music -- American thug" , "Worthless person; seventies rock style" , "Aggressive style of rock" .)
'a foolish person' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nitwit that I've seen before include "Silly fool" , "Fool, idiot" , "Informally a stupid person, like funny flea?" , "Twin it with a right eejit" , "Informally, a foolish person, featherbrain" .)