Be admitted to hospital department's emergency room (5)
I believe the answer is:
enter
'be admitted' is the definition.
(thesaurus)
'hospital department's emergency room' is the wordplay.
'hospital department' becomes 'ent'.
'emergency room' becomes 'er' (abbreviation popularized by US TV series).
'ent'+'er'='ENTER'
'to' is the link.
(Other definitions for enter that I've seen before include "Record - stage direction - come in" , "Fruit drink" , "Infiltrate" , "Move into" , "Come/go in" .)