Erect foundations to accommodate old actors' entrance (5,4)
I believe the answer is:
stage door
'actors' entrance' is the definition.
(I've seen this before)
'erect foundations to accommodate old' is the wordplay.
'erect' says the letters should be written backwards.
'foundations' becomes 'roots' (root can mean something's origin or foundation).
'to accommodate' means one lot of letters goes inside another.
'old' becomes 'aged' (similar in meaning).
'roots' reversed gives 'stoor'.
'stoor' placed around 'aged' is 'STAGE-DOOR'.
(Other definitions for stage door that I've seen before include "What players have to go through" , "Actors' entrance" , "Actors' entrance/exit" , "Artist's entrance" , "Company access" .)