In backstreet, parent's waving arms in sacred place (9)
I believe the answer is:
transepts
'arms in sacred place' is the definition.
(arm-like parts of a cross-shaped church)
'in backstreet parent's waving' is the wordplay.
'in' means one lot of letters goes inside another.
'backstreet' becomes 'ts' ('st' backwards).
'waving' indicates an anagram (letters wave around).
'parents' with letters rearranged gives 'ransept'.
'ts' going around 'ransept' is 'TRANSEPTS'.
(Other definitions for transepts that I've seen before include "Sections of church" , "In cross-shaped church, these are at right angles to nave" , "parts of cathedrals" .)