Tips or Tools to help smoothly merge clips of same tune together ?
(trying to lengthen a short clip of music by repeating it, but frustrated with the obvious star and end sounds, definitely sounds “chopped”)
Using Audacity 2.3.3
fade in / fade out ? crossfade ?
BTW, is there a reason you are not on 2.4.2 ?
I hope this helps. 
Audacity 2.4.2 is available via the Audacity website: https://www.audacityteam.org/download/windows/
Audacity 2.4.2 includes two plug-ins for crossfading:
This is often the easier one to use: https://manual.audacityteam.org/man/crossfade_clips.html
This one is more flexible but a little more complex: https://manual.audacityteam.org/man/crossfade_tracks.html