Previously mentioned area, if so shabby, beginning to decay (9)
I believe the answer is:
aforesaid
'previously mentioned' is the definition.
(I've seen this before)
'area if so shabby beginning to decay' is the wordplay.
'shabby' indicates anagramming the letters.
'beginning to' suggests taking the first letters.
The first letter of 'decay' is 'd'.
'area'+'if'+'so'='areaifso'
'areaifso' is an anagram of 'aforesai'.
'aforesai'+'d'='AFORESAID'
(Other definitions for aforesaid that I've seen before include "Above-mentioned" , "earlier" , "previously referred to" , "The one previously mentioned" .)