Sweet enough to relieve Bach of his backache. (4)
I believe the answer is:
cake
'sweet' is the definition.
(I know that cake is a type of sweet)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cake that I've seen before include "Sweet food; become encrusted" , "It might have layers" , "Mixture of flour, sugar, eggs and fat" , "Item of food or soap" , "Set" .)