Excessive energy in course of action (5)
I believe the answer is:
steep
'excessive' is the definition.
(steep can mean exceedingly expensive)
'energy in course of action' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'course of action' becomes 'step' (I've seen this in another clue).
'e' inserted inside 'step' is 'STEEP'.
(Other definitions for steep that I've seen before include "over the top" , "Soak; expensive (price)" , "Lofty" , "Hard for cycling" , "pickle" .)