I want to separate out the piano accompaniment part

Is it possible to separate out piano accompaniment from a recording with violin and piano with Audacity? I want to ‘extract’ out just the violin part, and perhaps do some synthesis to the violin part only.

As a fuzzy rule Audacity can’t split a mixed performance into individual instruments or voices.

You can try Effect > Vocal Removal and Isolation. It’s designed for voices, but sometimes you can get lucky.