Warble during class, in German (4)
I believe the answer is:
sing
'warble' is the definition.
(I've seen this in another clue)
'during class in german' is the wordplay.
'during' indicates the answer is hidden within the clue.
'SING' is hidden in the letters of 'class in german'.
(Other definitions for sing that I've seen before include "Inform - carol" , "Croon" , "pipe" , "Grass" , "Celebrate" .)