Framework of church when applied to sister (7)
I believe the answer is:
chassis
'framework' is the definition.
(chassis is a kind of framework)
'church when applied to sister' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'when' becomes 'as'.
'applied to' means one lot of letters go next to another.
'sister' becomes 'sis' ('sis' can be a synonym of 'sister').
'ch'+'as'+'sis'='CHASSIS'
'of' acts as a link.
(Other definitions for chassis that I've seen before include "Vehicle's base frame" , "Vehicle structure" , "Body frame" , "Car's structural framework" , "Framework of motor vehicle" .)