Conscious campaign to stop atomic energy (5)
I believe the answer is:
aware
'conscious' is the definition.
(associated in meaning)
'campaign to stop atomic energy' is the wordplay.
'campaign' becomes 'war'.
'to stop' is an insertion indicator (inserted letters plug or stop a gap).
'atomic' becomes 'a' (eg in A-test, atomic test).
'energy' becomes 'e' (physics symbol as in E=mc2).
'a'+'e'='ae'
'war' placed into 'ae' is 'AWARE'.
(Other definitions for aware that I've seen before include "Informed; conscious" , "Acquainted with or conscious of" , "Knowing (of something)" , "Bearing in mind" , "Cognizant" .)