Severe reduction in atomic energy (5)
I believe the answer is:
acute
'severe' is the definition.
('acute' can be a synonym of 'severe')
'reduction in atomic energy' is the wordplay.
'reduction' becomes 'cut' (cut is a kind of reduction).
'in' is an insertion indicator.
'atomic' becomes 'a' (eg in A-test, atomic test).
'energy' becomes 'e' (physics symbol as in E=mc2).
'a'+'e'='ae'
'cut' put inside 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Shrewd; severe" , "feature at the beginning and end of a French summer" , "Urgently pressing" , "Grave" , "Sharp; severe" .)