Additional time welcomed by girl left outside union (12)

I believe the answer is:

extramarital

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'outside union' is the definition.
(I've seen this before)

'additional time welcomed by girl left' is the wordplay.
'additional' becomes 'extra' (extra means additional or surplus).
'time' becomes 'am'.
'welcomed' means one lot of letters goes inside another (inserted letters are welcomed in).
'by girl left' becomes 'rital' (I can't explain this - if you can you should believe this answer much more).
'extra' enclosing 'am' is 'extrama'.
'extrama'+'rital'='EXTRAMARITAL'

Can you help me to learn more?

(Other definitions for extramarital that I've seen before include "outside union" , "Occurring outside marriage" .)

I've seen this clue in The Times.

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