Horse has half gone – whoa! (3)
I believe the answer is:
gee
'horse has half gone whoa' is the definition.
I can't judge whether this definition defines the answer.
'horse has half gone' is the wordplay.
'horse' becomes 'geegee' (child's name for a horse).
'has half gone' suggests halving the letters.
'geegee' with half the letters taken is 'GEE'.
Can you help me to learn more?
(Other definitions for gee that I've seen before include "Alphabet's seventh letter" , "- - whiz! (US)" , "Well I never" , "American exclamation" , "G" .)