Sharp causing laceration in casualty (5)
I believe the answer is:
acute
'sharp' is the definition.
(I know that sharp can be written as acute)
'laceration in casualty' is the wordplay.
'laceration' becomes 'cut' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'casualty' becomes 'ae' (abbreviation for Accident and Emergency).
'cut' inserted into 'ae' is 'ACUTE'.
'causing' is the link.
Can you help me to learn more?
(Other definitions for acute that I've seen before include "far from chronic" , "quick on the uptake" , "Extremely sharp or intense" , "Sharp; type of accent" , "Excruciating" .)