Two letters for a horse (3-3)
I believe the answer is:
gee-gee
'horse' is the definition.
(gee-gee is a kind of horse)
'two letters' is the wordplay.
'two letters' can mean two substitutions for 'letter'.
'letter' becomes 'gee' (G).
'letter' becomes 'gee' (G).
'gee'+'gee'='GEE-GEE'
'for a' is the link.
(Other definitions for gee-gee that I've seen before include "Horse (slang)" , "Child's word for a horse" , "Child's repetitive name for horse" .)