Church room exceptionally entered by short street (6)
I believe the answer is:
vestry
'church room' is the definition.
(I've seen this before)
'exceptionally entered by short street' is the wordplay.
'exceptionally' becomes 'very' (I've seen this in another clue).
'entered by' means one lot of letters goes inside another (I've seen this in other clues).
'short street' becomes 'st' (abbreviation; 'short' suggests a short form).
'very' enclosing 'st' is 'VESTRY'.
(Other definitions for vestry that I've seen before include "Small room attached to church" , "Office room in church" , "meeting of parishioners" , "Church room where clergy dress themselves" , "Part of church used as an office" .)