Girl's special union to last with princess (7)

I believe the answer is:

suzanne

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'girl's' is the definition.

'special union to last with princess' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'to last with' becomes 'z' (I am not sure about this - if you are sure you should believe this answer much more).
'princess' becomes 'anne' (I've seen this before).
's'+'u'+'z'+'anne'='SUZANNE'

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I've seen this clue in the Sydney Morning Herald.

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