Politics ultimately in moral decline (7)
I believe the answer is:
descent
'decline' is the definition.
(I know that decline can be written as descent)
'politics ultimately in moral' is the wordplay.
'ultimately' says to take the final letters.
'in' is an insertion indicator.
'moral' becomes 'decent'.
The last letter of 'politics' is 's'.
's' put inside 'decent' is 'DESCENT'.
(Other definitions for descent that I've seen before include "Slope leading from top" , "that concerns genealogist?" , "A climb down" , "Bloodline" , "Pedigree" .)