Ceremony is incorporated into plan by Brussels (9)
I believe the answer is:
eucharist
'ceremony' is the definition.
(I know that Eucharist is a type of sacrament)
'is incorporated into plan by brussels' is the wordplay.
'incorporated into' is an insertion indicator.
'plan' becomes 'chart' (chart can mean a plan or diagram).
'by' means one lot of letters go next to another.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'is' placed into 'chart' is 'charist'.
'charist' after 'eu' is 'EUCHARIST'.
(Other definitions for eucharist that I've seen before include "church service" , "host" , "Holy Communion" , "Bread and wine" , "Consecrated bread" .)