Go fast? Ridiculous rate! (4)
I believe the answer is:
tear
'go fast?' is the definition.
(I've seen this in another clue)
'ridiculous rate' is the wordplay.
'ridiculous' is an anagram indicator.
'rate' with letters rearranged gives 'TEAR'.
(Other definitions for tear that I've seen before include "Rent - drop" , "Rip, rend" , "Rip apart; drop" , "Secretion of the lacrymal glands" , "Come apart" .)