Penetrating wound seen in A&E (5)
I believe the answer is:
acute
'penetrating' is the definition.
('acute' can be a synonym of 'penetrating')
'wound seen in ae' is the wordplay.
'wound' becomes 'cut' (cut is a kind of wound).
'seen in' is an insertion indicator.
'cut' going inside 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Extremely sharp or intense" , "Shrewd; severe" , "far from chronic" , "Severe, penetrating" , "Grave" .)