To repeat it time after time requires energy (7)
I believe the answer is:
iterate
'to repeat' is the definition.
('iterate' can be a synonym of 'repeat')
'it time after time requires energy' is the wordplay.
'time' becomes 'era' (I've seen this before**).
'after' says to put letters next to each other.
'time' becomes 't'.
'requires' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mcr).
'it'+'era'+'t'+'e'='ITERATE'
(Other definitions for iterate that I've seen before include "State repeatedly, say again" , "Repeat, state again" , "Utter or perform repeatedly" , "Say again, repeat" , "Recapitulate" .)