Invalidate second and third of marriages with a ring (7)

I believe the answer is:
annular
'invalidate second and third of marriages with a ring' is the definition.
I can't tell whether this defines the answer.
'invalidate second and third of marriages with a' is the wordplay.
'invalidate second' becomes 'annul' (synonyms. I am not sure about the 'second' bit.).
'and' says to put letters next to each other.
'third of marriages' becomes 'r' (nrd letter of 'marriages').
'with' means one lot of letters go next to another.
'r' after 'a' is 'ar'.
'annul' next to 'ar' is 'ANNULAR'.
Can you help me to learn more?
(Other definitions for annular that I've seen before include "Doughnut-shaped -- a LAN run (anag)" , "Ring-shaped" , "in a band" , "Circular" , "Shaped like a ring" .)
