Union formed in grim area (8)

I believe the answer is:

marriage

I'm a little stuck... Click here to teach me more about this clue!

'union' is the definition.
(synonyms)

'in grim area' is the wordplay.
'grim area' anagrams to 'MARRIAGE'.
But, I'm unsure how the anagram is indicated.

'formed' is the link.

Can you help me to learn more?

(Other definitions for marriage that I've seen before include "Wedding" , ""Till death do us part" arrangement" , "Matrimony" , "Church occasion" , "Wedlock of alliance" .)

I've seen this clue in The Sun.

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