This convention is full of people, say (4)
I believe the answer is:
pact
'this convention' is the definition.
(a pact is a convention or agreement)
'full of people say' is the wordplay.
'full of people' becomes 'packed' (eg a packed restaurant).
'say' shows a homophone (sound like).
'packed' is a homophone of 'PACT'.
'is' is the link.
(Other definitions for pact that I've seen before include "Formal agreement between parties or states" , "Agreement, covenant" , "Treaty, formal agreement" , "Alliance" , "Settlement" .)