To repeat play one song from a playlist or from the entire FrostWire Library, click the Repeat Songs button twice. You should see a little '1' icon show over the button. Once the player finishes playing the current track, it will start playing it form the beginning once again. The player will loop the playback for you until you change the playback mode.
How to repeat the entire playlist when I play music from the FrostWire Library?
How to shuffle music in my playlist or entire FrostWire Library?
How to play music from my playlist in sequential order from the FrostWire Library?