Brief decline into deficit (9)
I believe the answer is:
shortfall
'deficit' is the definition.
(I know that deficit can be written as shortfall)
'brief decline' is the wordplay.
'brief' becomes 'short' (synonyms).
'decline' becomes 'fall' (synonyms).
'short'+'fall'='SHORTFALL'
'into' acts as a link.
(Other definitions for shortfall that I've seen before include "Deficiency" , "Not quite what's expected" , "what we're lacking" , "Not enough money" , "lack" .)