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