Severe reduction in A & E (5)
I believe the answer is:
acute
'severe' is the definition.
('acute' can be a synonym of 'severe')
'reduction in a e' is the wordplay.
'reduction' becomes 'cut' (cut is a kind of reduction).
'in' means one lot of letters goes inside another.
'a'+'e'='ae'
'cut' put within 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Severe; French accent" , "Perspicacious" , "Discerning" , "Sharp or crucial" , "Sharp-pointed - keenly perceptive" .)