"being edited" or "in process of editing" or "in edit" in one word

There is a term with less than six letters - Dirty; in this sense it's invariably used in data management as a flag to indicate an unsaved modified state.

Dirty bits can also be used in Incremental computing by marking segments of data that need to be processed or have yet to be processed. This technique can be used with delayed computing to avoid unnecessary processing of objects or states that have not changed. When the model is updated (usually by multiple sources), only the segments that need to be reprocessed will be marked dirty. Afterwards, an algorithm will scan the model for dirty segments and process them, marking them as clean. This ensures the unchanged segments are not recalculated and saves processor time. (-- Wikipedia)

For online content management systems, the usual idiom is 'checked out'/'checked in'.

An item is 'checked out' when a user is editing it; it is 'checked in' when they have finished editing it and it is available to other people for editing.

Users sometimes find this meaning a bit opaque, but an analogy to library books usually clears things up for them.