It's crazy to take a horse out of a race! (5)
I believe the answer is:
batty
'it's crazy' is the definition.
(I've seen this in another clue)
'a horse out of a race' is the wordplay.
'a horse' becomes 'bay' (bay is a kind of horse).
'out of' indicates putting letters inside (some letters on the outside of others).
'a race' becomes 'tt'.
'bay' going around 'tt' is 'BATTY'.
'to take' is the link.
(Other definitions for batty that I've seen before include "As mad as a mouselike mammal" , "not sound" , "Seemingly descriptive of vampire" , "Dotty!" , "Eccentric" .)