Talk during commemoration at tercentenary (6)
I believe the answer is:
natter
'talk' is the definition.
(I have seen 'Casual talk' mean 'natter' so perhaps 'talk' could also mean 'natter')
'during commemoration at tercentenary' is the wordplay.
'during' indicates the answer is hidden within the clue.
'NATTER' is hidden in the letters of 'commemoration at tercentenary'.
(Other definitions for natter that I've seen before include "Talk idly" , "Talk socially about nothing important" , "Chat away informally" , "Chat busily" , "Talk inconsequentially" .)