Advantage of four-speed gears (4)
I believe the answer is:
edge
'advantage' is the definition.
(I've seen this before)
'of four-speed gears' is the wordplay.
'of' says the answer is hidden in the clue.
'EDGE' is hidden within 'four speed gears'.
(Other definitions for edge that I've seen before include "Move little by little to the border" , "Intense quality; limit" , "'Margin, rim (4)'" , "Rim or sharp side of knife" , "Border or cutting side of knife" .)