For starters, eat oysters? Never in a thousand million years (3)
I believe the answer is:
eon
'a thousand million years' is the definition.
(I've seen this in another clue)
'for starters eat oysters? never' is the wordplay.
'for starters' suggests taking the first letters.
The initial letters of 'eat oysters never' is 'EON'.
'in' acts as a link.
(Other definitions for eon that I've seen before include "Major division of geological time" , "Vast age (alt. sp.)" , "In time, long division" , "Era" , "thousand million years" .)