Reduce high speed (4)
I believe the answer is:
clip
I believe this clue is a double definition.
'reduce' is the first definition.
(I know that clip is a more specific form of the action shorten)
'high speed' is the second definition.
(clip can mean a rapid rate of movement)
(Other definitions for clip that I've seen before include "Magazine" , "Cut short; cartridge holder" , "Short film extract; fasten" , "Cut - piece of film" , "Film extract; light blow" .)