Severe injury in A & E (5)
I believe the answer is:
acute
'severe' is the definition.
('acute' can be a synonym of 'severe')
'injury in a e' is the wordplay.
'injury' becomes 'cut' (cut is a kind of injury).
'in' means one lot of letters goes inside another.
'a'+'e'='ae'
'cut' placed inside 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Angle of less than 39 degrees" , "Sharp or crucial" , "Extremely sharp or intense" , "Short and severe" , "Sharp; severe" .)