Old cracks reflected heritage part of theatre (5,4)
I believe the answer is:
stage door
'part of theatre' is the definition.
(I've seen this before)
'old cracks reflected heritage' is the wordplay.
'old' becomes 'aged' (similar in meaning).
'cracks' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'reflected' is a reversal indicator (letters reflected back).
'heritage' becomes 'roots' (I've seen this in another clue).
'roots' back-to-front is 'stoor'.
'aged' placed inside 'stoor' is 'STAGE-DOOR'.
(Other definitions for stage door that I've seen before include "What players have to go through" , "Artist's entrance" , "Actors' entrance" , "Theatre rear entrance" , "Actors' entrance/exit" .)