Sport, an energy-filled fight (4)
I believe the answer is:
wear
'sport' is the definition.
(sport can mean to wear or display on one's person)
'energy-filled fight' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mca).
'filled' means one lot of letters goes inside another.
'fight' becomes 'war' (warring is a kind of fighting).
'e' inserted inside 'war' is 'WEAR'.
'an' acts as a link.
(Other definitions for wear that I've seen before include "Become damaged by use" , "Durham river" , "Clothing; degrade" , "Durham's river - erosion" , "Durability" .)