Crucial economy inside emergency department (5)
I believe the answer is:
acute
'crucial' is the definition.
(I've seen this in another clue)
'economy inside emergency department' is the wordplay.
'economy' becomes 'cut' (both can mean a cost-saving measure).
'inside' indicates putting letters inside.
'emergency department' becomes 'AE' (A and E, accident and emergency).
'cut' going into 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Urgently pressing" , "Discerning" , "Sharp; severe" , "Short and severe" , "less than right" .)