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Serious gap in astrology (5,4)


I believe the answer is:

black hole

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'astrology' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'serious gap' is the wordplay.
'serious' becomes 'black' (I can't justify this - if you can you should give a lot more credence to this answer).
'gap' becomes 'hole' (hole is a kind of gap).
'black'+'hole'='BLACK HOLE'

'in' is the link.

Can you help me to learn more?

(Other definitions for black hole that I've seen before include "Astronomical curiosity" , "'Region of space from which nothing can escape (5,4)'" , "Space left after collapse of a star" , "object in 21 [SPACE] having very strong 20 [GRAVITY]" , "discovered in Calcutta" .)

I've seen this clue in The Mirror.
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