Swears with energy becoming hostile (6)
I believe the answer is:
averse
'hostile' is the definition.
(thesaurus)
'swears with energy' is the wordplay.
'swears' becomes 'avers' ('aver' can be a synonym of 'swear').
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'avers'+'e'='AVERSE'
'becoming' acts as a link.
(Other definitions for averse that I've seen before include "Hostile" , "'Opposed, disinclined (6)'" , "A line of poetry is opposed" , "Disinclined - sounds like a poem" , "Not keen" .)