Singing in German, was deceitful to the bitter end (6)
I believe the answer is:
lieder
'singing in german' is the definition.
'was deceitful to the bitter end' is the wordplay.
'was deceitful' becomes 'lied' (I've seen this in another clue).
'end' indicates one should take the final letters.
The final letters of 'the bitter' are 'er'.
'lied'+'er'='LIEDER'
(Other definitions for lieder that I've seen before include "[COMPOSITION] s" , "German songs of the Romantic period" , "German Romantic songs for solo voice" , "numbers in German" , "works by Schubert" .)