Previously mentioned idea, so far baffling (9)
I believe the answer is:
aforesaid
'previously mentioned' is the definition.
(I've seen this before)
'idea so far baffling' is the wordplay.
'baffling' is an anagram indicator.
'idea'+'so'+'far'='ideasofar'
'ideasofar' with letters rearranged gives 'AFORESAID'.
(Other definitions for aforesaid that I've seen before include "Above-mentioned" , "earlier" , "previously referred to" , "The one previously mentioned" .)