It's crazy to tear at this speed! (4)
I believe the answer is:
rate
'this speed' is the definition.
(rate is a kind of speed)
'it's crazy to tear' is the wordplay.
'it's crazy' is an anagram indicator.
'tear' is an anagram of 'RATE'.
'at' is the link.
(Other definitions for rate that I've seen before include "Price - class - speed" , "The speed of movement" , "Find fault with - speed - charge" , "Assess or grade" , "Appreciate" .)