The undoing of Aunty, not needing a gin cocktail (7)
I believe the answer is:
untying
'the undoing' is the definition.
('untying' can be a synonym of 'undoing')
'aunty not needing a gin cocktail' is the wordplay.
'not needing' indicates named letters should be taken away.
'cocktail' indicates anagramming the letters.
'aunty' with 'a' taken out is 'unty'.
'gin' anagrammed gives 'ing'.
'unty'+'ing'='UNTYING'
'of' acts as a link.
(Other definitions for untying that I've seen before include "Loosening a knot" , "Releasing fastenings" , "Removing restrictions" , "Undoing, unfastening" .)