Prison in constant decline (4)
I believe the answer is:
cage
'prison' is the definition.
(cage is a kind of prison)
'constant decline' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'decline' becomes 'age' (get older).
'c'+'age'='CAGE'
'in' is the link.
(Other definitions for cage that I've seen before include "Put behind bars" , "Wired enclosure" , "In places squirrel's nest" , "Pen" , "Prison for 21" .)