Serious reduction in hospital department (5)
I believe the answer is:
acute
'serious' is the definition.
(thesaurus)
'reduction in hospital department' is the wordplay.
'reduction' becomes 'cut' (cut is a kind of reduction).
'in' indicates putting letters inside.
'hospital department' becomes 'ae' (A&E, Accident & Emergency).
'cut' put inside 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Incisive" , "Quick-witted" , "Short and severe" , "Sharp; severe" , "(Of an angle) less than 90 degrees" .)