Assume energy is required in battle (4)
I believe the answer is:
wear
'assume' is the definition.
(thesaurus)
'energy is required in battle' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'is required in' indicates putting letters inside.
'battle' becomes 'war' (I've seen this before).
'e' inserted within 'war' is 'WEAR'.
(Other definitions for wear that I've seen before include "___ and tear" , "Tolerate" , "brook" , "Become tiresome" , "Be clothed in" .)