Fast, rapid (6)
I believe the answer is:
speedy
I believe this is a double definition.
'fast' is the first definition.
(similar in meaning)
'rapid' is the second definition.
(I know that rapid can be written as speedy)
(Other definitions for speedy that I've seen before include "Rapid, fast" , "Fast-moving" , "in a hurry" , "Done quickly" , "in a fast car?" .)