The most common and most likely answer to this crossword clue is the 9 letter word ACCRETION
I've seen this crossword clue in Independent
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The straight part
We know that "abnormal union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case abnormal union = accretion.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 ACCRETION mean?
ACCRETION noun- The act of increasing by natural growth; especially the increase of organic bodies by the internal accession of parts; organic growth.
- The act of increasing, or the matter added, by an accession of parts externally; an extraneous addition
- Something added externally to promote growth the external growth of an item.
- Concretion; coherence of separate particles
- A growing together of parts naturally separate, as of the fingers or toes.
- The gradual increase of land by deposition of water-borne sediment.
- The adhering of property to something else, by which the owner of one thing becomes possessed of a right to another; generally, gain of land by the washing up of sand or sail from the sea or a river, or by a gradual recession of the water from the usual watermark.
- Gain to an heir or legatee; failure of a coheir to the same succession, or a co-legatee of the same thing, to take his share percentage.