Say, that's quick! (7)
I believe the answer is:
express
I believe this clue is a double definition.
'say' is the first definition.
(to express something is to say it)
'quick' is the second definition.
(I've seen this before)
'that's' acts as a link.
(Other definitions for express that I've seen before include "This train doesn't stop" , "Utter; without stopping" , "Fast - state - train" , "Quick -- exact" , "Specific; going at speed" .)