Gosh, which half of the horse is it?
I believe the answer is:
gee
'gosh' is the definition.
(I've seen this before)
'half of the horse is it?' is the wordplay.
'half of' indicates taking half.
'the horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'is it?' becomes 'ee' (I can't explain this - if you can you should believe this answer much more).
'gg' cut in half is 'g'.
'g'+'ee'='GEE'
'which' is the link.
Can you help me to learn more?
(Other definitions for gee that I've seen before include "crumbs" , "Gosh" , "Goodness" , "- - whiz! (US)" , "Well I never" .)