Brief decline into a black hole (9)
I believe the answer is:
shortfall
'a black hole' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'brief decline' is the wordplay.
'brief' becomes 'short' (similar in meaning).
'decline' becomes 'fall' (synonyms).
'short'+'fall'='SHORTFALL'
'into' acts as a link.
Can you help me to learn more?
(Other definitions for shortfall that I've seen before include "Loss" , "what we're lacking" , "Not quite what's expected" , "lack" , "Not enough money" .)