Swiftest (9)
'swiftest' is the definition.
('fast' can be similar in meaning to 'swift')
This is the entire clue.
(Other definitions for fastest that I've seen before include "Speediest" , "Most rapid" , "Quickest" , "Finishing first" .)
(Other definitions for speediest that I've seen before include "Fastest" , "Most rapid" .)