Go in emergency room below hospital department (5)
I believe the answer is:
enter
'go in' is the definition.
(synonyms)
'emergency room below hospital department' is the wordplay.
'emergency room' becomes 'er' (abbreviation popularized by US TV series).
'below' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'hospital department' becomes 'ent'.
'er' put after 'ent' is 'ENTER'.
(Other definitions for enter that I've seen before include "Register formally as a participant" , "submit?" , "Enlist, join up" , "A key on the right" , "Break in" .)