Audacity currently doesn’t allow recording a new segment within an existing clip in a track. To insert new material within a clip the user must record it separately in another track, then copy/paste that material to the destination track.
It would be very useful for efficiency to be able to record new material directly within an existing clip, analogously to inserting new text within an existing line when word processing.
A significant difference is that in a word processor document it is OK for text after the insertion point to shift to the right to make room for the new text. If Audacity did that in a project with more than one track, everything beyond the insertion point would be pushed out of sync with the other tracks.
In most cases there is no need to put the new clip into the first track. It’s normal to have multiple tracks in a project.
I narrate for blind listeners and record several stores per project, one per track. The new material for a given story needs to be inserted in the appropriate track.