Urge one to go crazy about a horse? (3,2)
I believe the answer is:
egg on
'urge' is the definition.
(to egg someone on is to urge them to do something)
'one to go crazy about a horse?' is the wordplay.
'to go crazy' indicates an anagram (a crazy ordering of the letters).
'about' indicates putting letters inside.
'a horse?' becomes 'gg' (geegee or GG is a childish term for a horse).
'one' anagrammed gives 'eon'.
'eon' enclosing 'gg' is 'EGG ON'.
(Other definitions for egg on that I've seen before include "Fire" , "Prompt" , "Urge, impel" , "Encourage to act" , "Prod" .)