Knows someone the definition (source) of the word marker?

NARKER is a FORTH 2012 standard word: https://forth-standard.org/standard/core/MARKER
It creates a word that, when executed, FORGETs itself and all words following.

It should be possible to define MARKER in FIG to create a word that passes it's own address into the middle of the FORGET logic. I haven't done this yet.

Another alternative is to change your program to delete MARKER & use FORGET where the MARKER created word would be called.