The most common and most likely answer to this crossword clue is the 8 letter word BACKDATE
I've seen this crossword clue in Independent
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More clues leading to the result BACKDATE
Buddy explains!

The straight part
We know that "Give retrospective validity to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case give retrospective validity to = backdate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BACKDATE mean?
BACKDATE noun- An assigned date that is earlier than the current or true date.
- To give or assign a date to a document that is earlier than the current or true date.