Melodious bird (4)
'melodious bird' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lark that I've seen before include "Frolic - songbird" , "bit of mischief" , "Bird; prank" , "Early riser" , "Singing bird and piece of harmless mischief" .)
(Other definitions for nightingale that I've seen before include "Bird noted for its song" , "Flier" , "Bird with beautiful song" , "Woman of note" , "Singer in London square" .)