The most common and most likely answer to this crossword clue is the 12 letter word IRREVERSIBLE
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Like Wham or Bananarama but not Abba or A-ha" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like wham or bananarama but not abba or a-ha = irreversible.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 IRREVERSIBLE mean?
IRREVERSIBLE adjective- Incapable of being reversed or turned about or back; incapable of being made to run backward.
- Incapable of being reversed, recalled, repealed, or annulled.
- Incapable of being reversed to the original state without consumption of free energy and increase of entropy.