Serious injury from blade brought into AE (5)
I believe the answer is:
acute
'serious' is the definition.
(acute can mean serious in a medical sense)
'injury from blade brought into ae' is the wordplay.
'injury from blade' becomes 'cut' (eg cut from a knife).
'brought into' indicates putting letters inside.
'cut' going into 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Shrewd; severe" , "Shrewd; serious" , "Piercing" , "quick on the uptake" , "less than right" .)