Finally, Standard and Poor's reversed decline (5)
I believe the answer is:
droop
'decline' is the definition.
(thesaurus)
'finally standard and poor's reversed' is the wordplay.
'finally' says to take the final letters.
'and' means one lot of letters go next to another.
'reversed' says the letters should be written in reverse.
The last letter of 'standard' is 'd'.
'poor' written backwards gives 'roop'.
'd'+'roop'='DROOP'
(Other definitions for droop that I've seen before include "Become limp" , "sink" , "Hang down loosely" , "be despondent" , "Sag, hang down" .)