Clue: Parent of boy who's a rascal? (3)
Answer: GUN
Explanation: Edit: Hovis has a better explanation – the parent of a 'son of a gun'="rascal", with 'son' more directly justifying the use of "Parent" and "boy"*
 cryptic def: a 'young gun' as a "boy who's a rascal", and their parent would be a [grown] GUN