This achievement can be earned after unlocking expeditions in your homebase. Continue through these side missions until you unlock Government Plane Wreck, which is the 5th expedition.
Once spawned in this level, you will want to head for the plane wreck which is in two pieces. Your main objective may be marked in the middle section of the plane, but we want to instead locate the front part of the plane. Once nearby, clear the area of any enemies, and feel free to explore for some journal entries and Splinter Cell lore.
In order to unlock the case with Sam Fisher’s suit, you will want to interact with the laptop on the desk in the main area on the plane, which will activate the large screen in front of you, and open the case. You can now go upstairs to claim the outfit, and finish the level if you’d like to equip it afterwards.