Passerine songbird (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
wagtail
(Other definitions for wagtail that I've seen before include "Slender songbird - a law git (anag)" , "singer" , "Songbird - gala wit (anag)" , "'Bird, its posterior projection jerks up and down (7)'" , "Common bird" .)