Heal mongrel with energy (4)
I believe the answer is:
cure
'heal' is the definition.
(I know that heal can be written as cure)
'mongrel with energy' is the wordplay.
'mongrel' becomes 'cur' (synonyms).
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cur'+'e'='CURE'
(Other definitions for cure that I've seen before include "Restore to good health" , "Heal or preserve" , "Preserve by smoking - make better" , "Make better - smoke" , "Foreign priest" .)