Chaotic worsening situation in underwater system, everything beyond belief, ultimately (4,4)
I believe the answer is:
free fall
'chaotic worsening situation' is the definition.
(something in free fall is declining rapidly)
'underwater system everything beyond belief ultimately' is the wordplay.
'underwater system' becomes 'reef' (as in a coral reef).
'everything' becomes 'all' (all things).
'beyond' is a charade indicator (letters next to each other).
'ultimately' indicates one should take the final letters.
The final letter of 'belief' is 'f'.
'reef'+'all'='reefall'
'reefall' after 'f' is 'FREE FALL'.
'in' acts as a link.
(Other definitions for free fall that I've seen before include "Motion under gravity" , "Downward movement under the force of gravity" , "Uncontrolled descent" , "Unchecked descent" , "Descent without parachute" .)