Not directly in the presidential powers. trump would need to appoint an Attorney General who then could fire Smith. Nixon did the same thing.
"Fully constitutional" is a bit of an exaggeration. Interfering with a criminal investigation to cover up a crime is covered under impeachment. It would be the duty of the House to impeach trump, and then the duty of the Senate to convict trump, if he were to do such a thing. Of course Republicans in Congress have been ignoring their constitutional duties lately, so trump might be able to get away with this.