Marriage completed, one changing residence? (5)
I believe the answer is:
mover
'one changing residence?' is the definition.
'marriage completed' is the wordplay.
'marriage' becomes 'm' (abbreviation in official records).
'completed' becomes 'over' ('over' can be a synonym of 'completed').
'm'+'over'='MOVER'
(Other definitions for mover that I've seen before include "One who is in motion - may also be a shaker" , "someone who brings forward proposal" , "Instigator, organiser" , "Dynamic type" , "Formal proposer" .)