Because of fast talk! (3)
I believe the answer is:
cos
'because' is the definition.
(short for 'because')
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cos that I've seen before include "Casual as" , "holiday destination" , "providing unsatisfactory explanation?" , "foreign island" , "Lettuce with long crisp leaves" .)