Melodious songbird (4)
'melodious songbird' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nightingale that I've seen before include "One singing sweetly after dark" , "Flier" , "Woman of note" , "Subject of 11 [ODE]" , "Bird - famous nurse" .)
(Other definitions for lark that I've seen before include "flier" , "Foolish activity, songbird" , "Early bird with morning song" , "Songbird - bit of fun" , "Bird; prank" .)