Citadel affected one key discussion (9)
I believe the answer is:
dialectic
'discussion' is the definition.
(thesaurus)
'citadel affected one key' is the wordplay.
'affected' is an anagram indicator.
'one' becomes 'i' (Roman numeral).
'key' becomes 'c' (musical key).
'citadel' anagrammed gives 'dialect'.
'dialect'+'i'+'c'='DIALECTIC'
(Other definitions for dialectic that I've seen before include "Pertaining to discourse" , "Disputation" , "Art of discussing truth of opinions" , "Relating to discourse" , "Debate to resolve conflicts" .)