Warble or croon (4)
I believe the answer is:
sing
I believe this is a double definition.
'warble' is the first definition.
(I know that warble is a more specific form of sing)
'croon' is the second definition.
(I know that croon is a more specific form of sing)
(Other definitions for sing that I've seen before include "Chorus" , "Make melodious sounds with voice" , "Grass" , "Perform (a song)" , "pipe" .)