Uniting against virus: hospital department (9)
I believe the answer is:
confluent
'uniting' is the definition.
(thesaurus)
'against virus hospital department' is the wordplay.
'against' becomes 'con' (as in pros and cons, for and against).
'virus' becomes 'flu' (influenza).
'hospital department' becomes 'ENT' (abbreviation for Ear, Nose and Throat department).
'con'+'flu'+'ent'='CONFLUENT'
(Other definitions for confluent that I've seen before include "Joining" , "Flowing together" , "Running into one" .)