Gain access to part of hospital – emergency room (5)
I believe the answer is:
enter
'gain access' is the definition.
(I've seen this before)
'part of hospital emergency room' is the wordplay.
'part of hospital' becomes 'ent' (I've seen this in other clues).
'emergency room' becomes 'er' (abbreviation popularized by US TV series).
'ent'+'er'='ENTER'
'to' acts as a link.
(Other definitions for enter that I've seen before include "Enroll, sign up" , "Treen (anag.)" , "Fruit drink" , "Break in" , "Enlist, join up" .)