Fake energy? Pity (5)
I believe the answer is:
shame
'pity' is the definition.
(synonyms)
'fake energy?' is the wordplay.
'fake' becomes 'sham' (synonyms).
'energy?' becomes 'e' (physics symbol as in E=mc2).
'sham'+'e'='SHAME'
(Other definitions for shame that I've seen before include "pity!" , "show up" , "scandal" , "disgraceful" , "Disgrace, ignominy" .)