Immersion in water corrupted bitmaps (7)
I believe the answer is:
baptism
'immersion in water' is the definition.
'corrupted bitmaps' is the wordplay.
'corrupted' is an anagram indicator.
'bitmaps' with letters rearranged gives 'BAPTISM'.
(Other definitions for baptism that I've seen before include "Initiation ceremony" , "Church admission rite" , "Christening ceremony" , "Religious rite involving water" , "Christian sacrament signifying rebirth" .)