Donkey isn't moving with energy — a hard problem (6,5)
I believe the answer is:
kidney stone
'a hard problem' is the definition.
'donkey isn't moving with energy' is the wordplay.
'moving with' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'donkey'+'isnt'='donkeyisnt'
'donkeyisnt' mixed up with 'e' can make 'KIDNEY-STONE'.
(Other definitions for kidney stone that I've seen before include "Renal calculus" , "Hard mass accumulating in organ" .)