Crazy brown horse eats turnip tops (5)
I believe the answer is:
batty
'crazy' is the definition.
(I've seen this before)
'brown horse eats turnip tops' is the wordplay.
'brown horse' becomes 'bay' (reddish-brown horse coat colour).
'eats' is an insertion indicator.
'turnip tops' becomes 'tt' (this might be a standard abbreviation which I don't know about).
'bay' enclosing 'tt' is 'BATTY'.
Can you help me to learn more?
(Other definitions for batty that I've seen before include "not sound" , "out to lunch" , "As mad as a mouselike mammal" , "Nuts" , "Dotty!" .)