Greek character getting easy to understand type of energy (7)
I believe the answer is:
nuclear
'type of energy' is the definition.
(I've seen this before)
'greek character getting easy to understand' is the wordplay.
'greek character' becomes 'nu' (letter from the Greek alphabet).
'getting' is a charade indicator (letters next to each other).
'easy to understand' becomes 'clear' (I've seen this before).
'nu'+'clear'='NUCLEAR'
(Other definitions for nuclear that I've seen before include "Relating to atomic energy" , "Type of energy" , "sort of family" , "Sort of energy" , "An ulcer (anag)" .)