Church, unaltered, containing second structure (7)
I believe the answer is:
chassis
'structure' is the definition.
(chassis is a kind of structure)
'church unaltered containing second' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'unaltered' becomes 'asis' (I've seen this in other clues).
'containing' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'asis' enclosing 's' is 'assis'.
'ch'+'assis'='CHASSIS'
(Other definitions for chassis that I've seen before include "basic structure" , "Car body" , "Car's body" , "Base-frame of car" , "Body framework" .)