Lob chalk over, with energy, to astronomical phenomenon (5,4)
I believe the answer is:
black hole
'astronomical phenomenon' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'lob chalk over with energy' is the wordplay.
'over with' is an anagram indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'lob'+'chalk'='lobchalk'
'lobchalk' anagrammed with 'e' gives 'BLACK HOLE'.
'to' is the link.
Can you help me to learn more?
(Other definitions for black hole that I've seen before include "Space left after collapse of a star" , "'Region of space from which nothing can escape (5,4)'" , "discovered in Calcutta" , "Astronomical curiosity" , "object in 21 [SPACE] having very strong 20 [GRAVITY]" .)