moves quickly (6)
'moves quickly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for darts that I've seen before include "Tears" , "They are thrown at targets in pubs" , "Traditional pub game" , "quick movements" , "Game; moves quickly" .)
(Other definitions for speeds that I've seen before include "Rates of acceleration" , "these may be in knots" , "Goes fast, maybe too fast" , "Rates of movement" , "Moves very quickly" .)
(Other definitions for motors that I've seen before include "Locomotives, machines" , "Mechanical devices with moving parts" , "They supply motion or power" , "Engines" , "Drives" .)