Begin to say "That's where I dined" (8)
I believe the answer is:
initiate
'begin' is the definition.
(I've seen this before)
'that's where i dined' is the wordplay.
'that's where' becomes 'init' (I can't explain this - if you can you should believe this answer much more).
'dined' becomes 'ate' (I've seen this in another clue).
'init'+'i'+'ate'='INITIATE'
'to say' acts as a link.
Can you help me to learn more?
(Other definitions for initiate that I've seen before include "Begin - teach" , "Make something begin" , "Bring up a topic or prepare the way for" , "Originate" , "Start, begin" .)