Matron initially exposed hospital entrance (5)
I believe the answer is:
mouth
'entrance' is the definition.
(thesaurus)
'matron initially exposed hospital' is the wordplay.
'initially' says to take the initial letters.
'exposed' becomes 'out' (I've seen this in other clues).
'hospital' becomes 'h'.
The initial letter of 'matron' is 'm'.
'm'+'out'+'h'='MOUTH'
(Other definitions for mouth that I've seen before include "River part - conversation opener?" , "chops" , "River outlet" , "trap" , "Oral cavity" .)