Fortune required to get substantial energy (4)
I believe the answer is:
fate
'fortune' is the definition.
(I know that fortune can be written as fate)
'to get substantial energy' is the wordplay.
'to get' says to put letters next to each other.
'substantial' becomes 'fat' (fat can mean large or substantial).
'energy' becomes 'e' (physics symbol as in E=mc2).
'fat' next to 'e' is 'FATE'.
'required' is the link.
(Other definitions for fate that I've seen before include "Predestiny" , "Final issue" , "Clotho, Lachesis or Atropos" , "E.g. Clotho, Atropos" , "Fortune" .)