The speaker's filled a convention (4)
I believe the answer is:
pact
'a convention' is the definition.
(thesaurus)
'the speaker's filled' is the wordplay.
'the speaker's' shows a homophone (sound like).
'filled' becomes 'packed' (packing is a kind of filling).
'packed' sounds like 'PACT'.
(Other definitions for pact that I've seen before include "deal" , "International treaty" , "Agreement, covenant" , "Written agreement between two states" , "Settlement" .)