Shift selection (4)
I believe the answer is:
gear
'shift selection' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gear that I've seen before include "Set of wheels working by engaging cogs" , "Toothed wheel that interlocks with another" , "Clothes -- paraphernalia" , "Toothed wheel which engages with another one" , "Informal clothes seen in rage" .)