In addition, energy is included in the cost (9)
I believe the answer is:
thereunto
'in addition' is the definition.
(I've seen this before)
'energy is included in the cost' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mcu).
'is included in' means one lot of letters goes inside another.
'cost' becomes 'runto' (I've seen this in another clue).
'the'+'runto'='therunto'
'e' put into 'therunto' is 'THEREUNTO'.
(Other definitions for thereunto that I've seen before include "To that (archaic)" , "Almost obsolete word meaning 'to that'; formally, 'in addition'" , "as well" , "to that place" , "To that (formal)" .)