Go fast: crazy rate! (4)
I believe the answer is:
tear
'go fast' is the definition.
(I've seen this in another clue)
'crazy rate' is the wordplay.
'crazy' indicates anagramming the letters.
'rate' with letters rearranged gives 'TEAR'.
(Other definitions for tear that I've seen before include "It's secreted by the lachrymal gland" , "Shred" , "Split - career" , "Rip - drop from eye" , "Rip, pull apart" .)