The last loud festival was ineffectual (6)
I believe the answer is:
effete
'ineffectual' is the definition.
(thesaurus)
'the last loud festival' is the wordplay.
'last' says to take the final letters.
'loud' becomes 'f' (short for forte - 'loud' in music).
'festival' becomes 'fete' (synonyms).
The last letter of 'the' is 'e'.
'e'+'f'+'fete'='EFFETE'
'was' acts as a link.
(Other definitions for effete that I've seen before include "Feeble, having lost vitality" , "Lacking in power" , "Feeble, having lost vitatlity" , "Affected, ineffectual" , "Feeble, lacking vitality" .)