Is there any single word for the meaning "Stealing something for remembrance". Or how well can we tell this.
The words normally used to mean something you take for remembrance are memento, souvenir, remembrance, or keepsake. None of these imply that they are taken by theft. (Google: [ define memento ])
The closest term I can think of is scalping, which is the taking of a battle trophy to memorialize one's bravery. That is stealing, you could say. But you could really only use it correctly if what you are taking is a scalp. (Wikipedia: Scalping)
Consequently, the simplest phrase might be "steal a souvenir".
Coin it if you will,
Though the clept root also exists, I would prefer not to suggest its use, to avoid confusion with cleft, which both spells and sounds very similar.
I have not tried to resolve the fact that the roots are from L and Gk, probably making this an invalid word formation, unlike biblioklept.