Fake source of energy? Disgraceful! (5)
I believe the answer is:
shame
'disgraceful' is the definition.
(I've seen this before)
'fake source of energy?' is the wordplay.
'fake' becomes 'sham' (sham can mean fake or pretend).
'source of' indicates taking the first letters (the start - or source - of the word).
The initial letter of 'energy' is 'e'.
'sham'+'e'='SHAME'
(Other definitions for shame that I've seen before include "Humiliation; dishonour" , "Disgrace - Salman Rushdie novel" , "scandal" , "Feeling of humiliation" , "disgraceful" .)