Regarding union input involved with gangster (7)

I believe the answer is:

nuptial

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'regarding union' is the definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.

'input involved with gangster' is the wordplay.
'involved' is an anagram indicator (involved can mean confusing or complex).
'with' says to put letters next to each other.
'gangster' becomes 'al' (Al Capone).
'input' anagrammed gives 'nupti'.
'nupti'+'al'='NUPTIAL'

Can you help me to learn more?

(Other definitions for nuptial that I've seen before include "Until pa (anag.)" , "To do with marriage or a wedding ceremony" , "Pertaining to weddings" , "Matrimonial" , "Like wedding ceremony" .)

I've seen this clue in The Guardian.

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