To invalidate, as a marriage (5)
I believe the answer is:
annul
'to invalidate as a marriage' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for annul that I've seen before include "Nullify" , "Declare (a law) invalid" , "Obligation" , "Cancel, say a law" , "Quash" .)