My, my! It's a horse! (3-3)
I believe the answer is:
gee-gee
'a horse' is the definition.
(gee-gee is a kind of horse)
'my my' is the wordplay.
'my' becomes 'gee' (I've seen this before).
'my' becomes 'gee' (I've seen this before).
'gee'+'gee'='GEE-GEE'
'it's' is the link.
(Other definitions for gee-gee that I've seen before include "Child's word for a horse" , "Horse (slang)" , "Child's repetitive name for horse" .)