Interval where sea god gains energy (7)
I believe the answer is:
tritone
'interval' is the definition.
(I know that tritone is a type of interval)
'sea god gains energy' is the wordplay.
'sea god' becomes 'triton' (figure of Greek myth).
'gains' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'triton'+'e'='TRITONE'
'where' acts as a link.
(Other definitions for tritone that I've seen before include "Augmented fourth" , "Musical interval" , "long interval?" .)