About marriage, when it turns belligerent (7)
I believe the answer is:
martial
'belligerent' is the definition.
(thesaurus)
'about marriage when it turns' is the wordplay.
'about' indicates putting letters inside.
'marriage' becomes 'm' (abbreviation in official records).
'when' becomes 'aral' (I can't explain this - if you can you should believe this answer much more).
'turns' shows that the letters should be reversed in order.
'it' backwards is 'ti'.
'm'+'aral'='maral'
'maral' going around 'ti' is 'MARTIAL'.
(Other definitions for martial that I've seen before include "These arts concern fighting sports" , "satiricist" , "Belligerent" , "associated with arts and law" , "poet of ancient times" .)