Some pretty petty sort! (4)
I believe the answer is:
type
'sort' is the definition.
(I know that type is a type of sort)
'some pretty petty' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'TYPE' is hidden in the letters of 'pretty petty'.
(Other definitions for type that I've seen before include "Print - classification" , "Kind - press keys" , "Class, kind or printed characters" , "Sort - way to write" , "Class or group of a particular class" .)